@Vree  
Derpy isn’t a Dash recolor like Daring. Different mane.

(PS. and of course in one episode, DW was a pegasus…  
)

@Psychopomf  
…Actually, just now, a pick that may actually work came to mind: Daring Do.  
I know, I know…I said repeating character models was bad (Derpy, Raindrops, DD all share Dash’s model, of course) but listen up.
 
With the Doctor departing, there is room opening up for another ‘adventure catalyst’ type character. Just like, for example, Donald Duck has Scrooge as an excuse to go on all sorts of epic quests, the Doctor is a reason for the Derpy gang to face all kinds of supernatural menaces. But when it is all said and done, the Doctor’s universe will always be crossover stuff, never quite fitting in with Equestrian canon.
 
Now, with Daring, the Bakground Six has a direct link to all of Equestria’s distant lands, forgotten magic and weird fauna.
 
In the end I’d probably do a round-robin thing when the 6th slot is filled with a different “guest” pony every stroy (though not neccessarily just Daring’s; I imagine it is not always to drag Octavia into a story that is set far from Ponyville, for example), so that there’s an excuse to give some spotlight to the other fan faves. But as the 6th regular I think Daring could work very well.
 
I may write that fanfic sometime, I sense potential in it. :D

@Vree
 
I could see that. Another vote for Blossomforth! …though Spitfire would be neat. Could have the dynamic of her taking time off with the Wonderbolts to hang with her friends. Like an older more responsible Rainbow Dash

We could swap the Doctor for a pegasus…If indeed there was a worthy candidate.  
Unfortunately, no other pegasus even comes close - all the other candidates-in-waiting on the popularity poll are wingless.
 
Blossomforth? To me she’s the most likely candidate. :D She’s got a unique enough look, enough popularity. Plus she was in Derpy’s ‘crew’ with Raindrops in Hurricane Flutershy. But in actual honesty, she can only boast a single show appearance.  
Screwball? (Well, she can fly…) More of a villain and fanon character really.  
Raindrops? Apart from appearing in a scene with Derpy once, she doesn’t have much going for her, she has negligible popularity and bg appearances or interesting fanon. Plus her using Derpy’s template (same hairstyle etc.) pretty much excludes her in itself.  
Wild Fire? But she’s a confirmed OC/persona, and doesn’t appear that much.  
Cloudchaser & Flitter? You can’t really have one without the other or both without exceeding the 6-pony limit.  
Dizzy Twister or Cloud Kicker or Sassaflash? (or one of the other regular bg pegasi) They don’t really have the popularity to back it up.  
Berry Punch? Without doubt, she is the next in line by popularity. Alas, she is an Earth pony.  
Alicorns? Nyx? Pluto/Erroria? Donut Steal?

@bluemeganium  
Oh sheesh, not the matriarchy thing again. There had better be a male alicorn somewhere by S5.

@Millennial Dan
 
Ewww male ponies. In matriarchal Equestria. Where stallions have no rights and the mares rule.
 
Raindrops > Time Turner. One of Derpy’s first shippings (1x16) and two pegasi.

@ClownDicks  
Well, that does seem to be the opinion of a large part of the fandom. Probably not a majority, though.

I think if I were to swap one out, it would be Bon Bon, since there’s really not much about even her fanon personality I find interesting. But, who would replace her? Raindrops? Heck if I know.

@The Twelfth Doctor Whoof  
If anything, John Smith Time Turner is like when Spike was the new Rainbow Dash, in that the sixth member of the team is both a guy and not a pony.

What’s wrong with having a stallion in there? Nothing at all.

@bluemeganium  
One error:
 
Mane Six: Two Pegasi, Two Earthponies, One Unicorn, and One Alicorn (if you consider the events of MMC and S4 to have actually happened)
 
Background Six: This is the most popular permutation, but I’ve seen The Doctor switched out for other mares of all 3 races to keep the all-female motif and/or racial balance.

@bluemeganium  
>two unicorns  
Don’t you mean one beautiful and unique snowflake, one unicorn, etc.?
 
I keed, I keed.

Two major errors:
 
Mane Six: two unicorns, two pegasi, two earth ponies. All of them female.  
Background Six: two unicorns, one pegasus, three earth ponies. One of the is male.
 
Incomparable!
 
WHY.jpg?

@Background Pony  
FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U FUK U

@FanOfMostEverything  
In Derpy’s case, it’s Mentally Defective.

Aw, nothing to do with the Mentally Advanced Series, it seems.