The Bold and the Beautiful.  
Generally an episode about the Badass Rarity which surfaced in S2 a few times
 
The other mane 5 get lost in the everfree forest and plot convenience prevents them from being found.
 
So Rarity must venture into the dark forest to find them.
 
I would make it a general rambo/action movie spoof with terribly cliched one liners.

Story arch throughout whole season.

It’s hard for me to write something without it taking a mature turn.

I’d write a luchadore wrestling episode about how Sunset Shimmer persuades the Mane Six to take part in a wrestling tournament.
 
Also Fluttershy demonstrates that her bear-massaging skills are hardcore enough to trounce the competition.

It would be about a single Changeling. No armies, no Chrysalis, no threats to Equestria. Maybe he/she participated in the battle in Canterlot during the Wedding Fiasco, but only under orders.
 
This Changeling moves to Ponyville, and simply wants to live a normal life. Only a few characters are willing to give it a chance, which leads to the Changeling nearly starving to death due to being unable to feed on love. In the end, said Changeling is accepted, and becomes a permanent fixture in Ponyville, alongside various other Supporting Characters (same level as Big Mac or Mayor Mare).

Episode in Manehatten  
theme: Basketball

Rarity episode, about her rise and fall as a crime lord.

A five-part epic Changeling war. Oh wait, I can’t write.

Celestia and Luna episode. With possibly Discord.