Brainstorming, organizing, collaborating, this thread is all about throwing stuff against the wall and seeing if it sticks. Post an idea you might want to see explored or claim one to write yourself. It's all fair game.

A few ideas gleamed from comments I've seen:

Why doesn't Trixie trust wheels? Could it have something to do with the friendly neighborhood shapeshifter calling himself that very word?

Rainbow Dash vs. the Scarecrow: Applejack has a new friend in town. Could he be the one to rid the farm of its pesky narcoleptic pegasus problem?

How Twilight met Stormy: The moment that started this entire universe has yet to be put to page. What exactly happened? And do we ship them?

Yes, yes we do.

Also, a changeling disguises as one of Rarity's Ponnequins and Sweetie catches him but can't convince Rarity.

5895787
Oh, that'd be a good one. Bring Sweetie Belle back into this.

5895797
Even better, she notices Apple Bloom doing way better than normal at school and, upon investigation, finds that a cute changeling colt is disguising as her bow and whispering answers into her ear.

5895818
Better indeed.

A changeling replaces a daughter who was killed in the Canterlot invasion to try and make her family happy again.

5897448 This would be an interesting idea—one that would probably need the [sad] tag, I think—but if the daughter was killed in the invasion, how would the parents be able to cope with the fact that one of the creatures that was the indirect cause of their daughter's death was now impersonating her? Wouldn't there be some lingering...hatred? For lack of a better word at the moment.
Just trying to help you with an important plot point so you can get started if you go through with the idea.

Good point. What if the changeling replaced a daughter who either died a naturel death or died violently, but not because changelings caused it? I just have a weak idea seed at the moment.

5898212
A good way to make a story's seed grow is to start answering questions about it. Stuff like, "Who would do this?" to get started on characterization.

One question I'd have for this would be, "How long does the changeling pose as the daughter?" If it's a full time thing... that's a little weird. My brother has kids, and I couldn't imagine him adopting another if one of them died and renaming him the same thing.

5895679 Can't be at your friend/family wedding, let us (the changeling) tack your place for the photos and other things

https://www.fimfiction.net/story/310047/15/twilight-sparkle-becomes-a-changeling-queen-book-1/chapter-15#comment/7211189

"have a changeling change into your deceased-lover" And/Or "have a changeling change into the pony that couldn't attend your wedding for the photos"
you know what I mean ?

https://www.fimfiction.net/story/310047/15/twilight-sparkle-becomes-a-changeling-queen-book-1/chapter-15#comment/7214366

"Do you have somewhere you don't want to be? Are you too busy to attend to everything you want to do? Then try our Changeling Body Double service! Nopony will know it's not the real you. We promise."

https://www.fimfiction.net/story/310047/15/twilight-sparkle-becomes-a-changeling-queen-book-1/chapter-15#comment/7214459

It's perfect if you think about it. High profile singer/other celebrity adored by the general populace gets to go home and relax while their body double gets all the adoration they can hold while doing the signing. Or the public appearance.
Modeling is also a career Changelings are made for. A single changeling can (in most interpretations) take the form of anyone, meaning they can model anything a designer comes up with.