Further options include:
 
1 Mirror Sombra (If you haven’t read enough IDW, then ignore this one)
 
2 Trixie, for being shown up at the battle of the bands (3rd place, ugh) And she has a track record of getting involved in magic she can’t control.
 
4 where’s 3?
 
5 The entire shadowbolt/crystal prep team. Because they are clearly a mirror party, but they never got to face off against the m6 properly, nothing like what a mirror party fight should be.
 
6 an OC, because then you can do what the gak you want, have them behave however you like, because it’s your character.
 
3 ahh here it is

abacus cinch and the dazzlings deadly alliance in the leyend of everfree  
It is quite obvious that it is cinch that to contacted adagio to offer an alliance and take revenge on sunset sci twilight the rainbooms and clear the princess pony twilight as seen cinch was lucky to to run into the device that sunset dropped it on purpose to destroy and not let anyone else use it and cinch he picked it up off the floor and tocommand repair with people who are interested in the subject of magic  
I see in this comic trouble sunset as for rainbooms and ultimately will blame a sci twilight for creating a device that condemned all magical creatures equestria including the princesses alicorn

@40kponyguy  
I’m sure you won’t, it looks great. Sorry, I’m all out of ideas.

@40kponyguy  
Wasn’t the mirror moved again, from the palace in Canterlot, to the Crystal Empire, to Twilight’s castle, the human world endpoint always staying in the same spot?

@Background Pony #586A  
Only reasoning I can come up with is, the mirror was moved. Therefore the point in the Equestria Girls world it connected to also moved.
 
It’s an idea I’ve only seen used three times.  
Here, in this comic.
 
Studio Ghibli’s ‘The Cat Returns’
 
And the old animated 80s series ‘Dungeons and Dragons’. (There’s an episode with a box that is basically a portal. Put this box in the right place and the group could use it to return home).

@40kponyguy  
I saw that part, but damned if it makes any sense. Why didn’t Twilight and Sunset end up in the same place? Why would it connect to a random statue instead?

@Background Pony #586A  
I suspect the Dazzlings don’t know about the portal, (if you go off the IDW comic they got dumped in an alleyway, so they wouldn’t know there was a portal outside the school).  

 
 
@BlazingDazzlingDusk  
I can only hope future strips do not disappoint.  
Remember, I’m looking for suggestions here. My idea for the story is fairly well laid out, but I need ideas to help bulk it out.

That last panel will haunt me o.o

It looks amazing so far, love Adagio’s expression, I can’t wait to see the rest!

@40kponyguy  
I’m never quite sure where they stand in terms of returning to Equestria. They might want to go back, but figure they could just be thrown into another dimension again whereas no such magic has shown itself in the new world, maybe they found the portal themselves (would only make sense that they knew where it was if they were banished through it) and couldn’t get through, like Starswirl hexed them to keep them from ever coming back, or maybe they’ve been there long enough that they’ve just given up hope of ever going home.
 
They all make it clear that they don’t like the human world, but maybe they know when to fold’em and figure they have better odds against the human world than the ponies and their unending supply of instant-win magic. (Little did they know, they were up against ponies anyway)

@Background Pony #586A  
My own take on the Dazzlings is Adagio being the leader because she’s better at going through with ideas. The other two might have ideas, but don’t have the nerve to go along with them.  
The other two just follow her lead.  
Aria didn’t really get enough character building in the film to make any firm decisions on. This could work in my favour for the story, as it gives me options for things which would seem out of character for the other two.  
Sonata, to me, is basically an evil blue version of Pinkie Pie. Ditzy, amusing, comedy relief, but there’s that subtle hint that she’s not as dumb as she seems.
 
As for the zapping Sunset in to Equestria, I was contemplating that being Sci Twi suggesting using the magic capture device to oppose the Dazzlings, but then going up on stage and instead revealing her nightmare form, and using the magic to zap Sunset.  
After that, maybe the Dazzlings trying to get Sci Twi to get the other girls, but Twi refusing as they are her friends. This would drive a rift between the Dazzlings and her.
 
Finally, I got the feeling that the Dazzlings didn’t want to go back to Equestria.  
They discovered Equestrian magic, and rather than “Oh, that must mean there’s a way home”, Adagio went “we can use this magic to make everyone love us here”.

I figure they’d work together to make the thing, but Adagio is very clearly the brains of the outfit. Through the whole movie, she’s the one making all the intelligent observations, constantly having to explain things to Aria and Sonata, always the one seizing opportunities to twist CHS to work for her through quick-witted Xanatos Speed-Chess as Aria and Sonata follow her lead. We may not know much about Aria, but that her reaction to “Do you know what this means?!” when they see magic again is just to shrug and mutter “I’unno,” the exact same reaction as Sonata, tells me she’s not the sharpest knife in the drawer. Not necessarily stupid, but not the smartest of the trio.
 
I’ve never bought the idea that Sonata was a secret genius. She spends the entire movie being a shallow ditz, so people think “Oh, she must be faking/pretending to be dumb!” just to give her some redeeming characteristic with the most basic “surprise” factor possible; just reversing what canon shows us. It’s the same as making Aria coo at bunnies because she’s “supposed to be the tough girl.” (Does she even do anything ‘tough’ through the whole movie…?) It’s not impossible; but it is something you’d have to just arbitrarily assign to their characters.
 
Personally, I like to think that the way the sirens work is that Adagio is the leader because she’s smarter, stronger, and has the burning drive to always get what she wants in the end, but Aria kept her grounded by always criticizing her plans in a “Remember, you’re only a man(siren)” kind of way, even if she couldn’t come up with anything substantial herself, or threw a tantrum when Adagio did the same thing to her.
 
Sonata may not be all that smart; but there’s a depth people usually overlook with that kind of character; being in touch with her feelings. It’s like Forest Gump, “I may not be a smart man, but I know what love is.” People say Sonata is ‘innocent’ (despite gleefully doing just as much evil as the other two) because she acts kind of like a child. I see that more as her being the Id to Aria’s Ego and Adagio’s Superego; Sonata being the one who knows what she wants, how she feels, and how other people feel without all the little distractions more mature minds have developed, the ones she just sees as getting in the way of her own happiness, using that to drain more power, even if Adagio decides the best use for it. The sirens feed on negative emotions, and if Sonata is better with empathy, she may be key to absorbing as much power as they do.
 
…Sorry for all my rambling. ^^;  
If you don’t agree with these interpretations, you’re more than free to use your own, but those are my thoughts.
 
I like the zap-to-Equestria idea for how much room it gives the sirens and maybe the Shadowbolts as opposed to more predictable hero dialogue, but why would they have such a thing and not use it to get back to Equestria themselves? Maybe Cinch or Sci-Twi (note: I don’t see her earnestly turning evil just for being ‘shown up,’ especially when she was the one that showed Sunset up in the competition) pushes Sunset through the portal and uses the spectrometer to seal it again to buy time?

@Background Pony #586A  
I’d prefer it if the story concludes with the status quo as it is at the moment (aside from the Dazzlings can actually sing competently without any powers).
 
As for the Crystal Prep relations, I haven’t fully decided there either.  
I know I want a scene with Sonata interacting with Soursweet.  
(Sonata’s relative innocence getting her on the receiving end of a Soursweet tantrum).
 
Getting a place at Crystal Prep as students could also be more easily explained if the mystery figure on this page is Cinch.  
If it is Sci Twi, then it makes more sense for the second magic capture device/spectrometer. (She could have made it and planted it back at her lab room, then told Adagio where to find it).
 
Also, which of the Dazzlings would be able to build a spectrometer, even if they had the plans?  
Aria? (The least well defined of the girls, and therefore we don’t know how smart she really is).  
Or Sonata? (A smart girl hidden behind the veil of a carefree idiot).
 
Another thing I want to incorporate is having either the Dazzlings, or Sci Twi zap Sunset Shimmer, dropping her back in Ponyville. This gives us the option of having the pony versions of the main 6 coming over to help beat the Dazzlings. Even more funny if those girls turn up when everything is concluded, and leaving the pony main 6 and Sunset wondering what the hell just happened.

Hm. Depends on whether you’re going with the standard “they lose no matter how much work they put in because fate just favors the good guys” thing, a ‘bad’ ending in which the villains win, but at least their fans are happy because it’s only fan-art/fanfiction anyway, or maybe something kind of inbetween, where they stop being a huge threat, but aren’t necessarily good guys, either.
 
Maybe they buddy up with the Shadowbolts. I can see Adagio getting along with Sugarcoat, both being the ones saddled with pointing out the obvious in their groups while Sour Sweet gets along with Sonata when she’s peppy, Aria when she’s mean, or something like that. Then Lemon comes back with the Rainbooms all “They’re right over here, they were doing evil magic and everything and you’ve gotta stop ‘em!!”, but then the sirens and Shadowbolts are just chilling together, Rainbooms giving Lemon annoyed looks as they figure this is just another Crystal Prep prank, go home, and leave Lemon very, very confused.
 
I’m also guessing that getting a third spectrometer (what the device is called) will be a plot point? They could feasibly find out about Sci-Twi, loot her little lab-room, find the notes Twilight made in building them, and make one themselves, the interval to which might be the period in which they grew to like the Shadowbolts, if you decide to do that.
 
I happen to have thought about the sirens getting their hands on the spectrometer before, as you might be able to tell. :)
 
Anyway, here’s hoping you can use any of that. Happy drawing!

@Background Pony #586A  
I’m open to suggestions.  
Still haven’t fully decided where this is going in the end yet.
 
So far I have Adagio gets the magic capture device, but it’s damaged.  
She also learns there’s another at Crystal Prep. (This page).
 
Then she needs to get Aria.
 
Third, she needs to get Sonata.
 
After that, they need to break in to Crystal Prep to get the undamaged magic capture device.  
Also need some way they can use that to collect energy to regain their powers.
 
Next, they need to recover their singing ability.  
(In this case, I’m going with them using a karaoke bar to practice, thereby they can sing without their gems this time. Could make for some funny scenes too with them winding each other up over bad scores).
 
Then they need to get some power back. Had the idea of having them going to Crystal Prep as students, seeing as they’re known at Canterlot High.  
This could make for some fun scenes interacting with the Shadowbolts.
 
In the long run, I want Lemon Zest to go to Canterlot high to get help. (As with Vinyl, constantly wearing headphones, so gets away from the Dazzling’s spell).
 
Need ideas to bulk this concept out.

@40kponyguy  
Ah, well that’s certainly a change! Forgive me for being jaded, but Sonata tends to get the best of everything as far as fans are concerned, because favoritism is such a huge part of the inspiration.
 
If this is actually something else, then I look forward to seeing where it goes! :D

@Background Pony #586A  
I’m thinking of having Sonata working at a fast food restraunt.  
Have Adagio saying something like “look at you. How far you’ve fallen”  
(Aria, under her breath, “it’s like she fell off the doorstep”)

I figure Aria wouldn’t need much convincing if she thought they could get their magic back. “Oh, I came to get you because I learned we have another shot, but if you’d rather sashay around a walkway for the amusement of these troglodytes, then I guess I can just make Sonata my second-in-command…”
 
Now, given the usual run of siren fan-works, Sonata might be harder to convince if she’s living in a mansion somewhere, or gone the usual route of being Super Best Pals with the Rainbooms.

Still not fully decided who Adagio is talking to here.
 
I have the idea it’s someone who wants to get payback on Sunset Shimmer.  
They also know of Crystal Prep.  
Thirdly, they’ve heard of the Dazzlings.
 
So, two real options.