I absolutely agree with you about what-ifs. They're simply the bread and butter of almost all great stories.
For 'The Three Sisters', it's what if Rarity was a changeling queen all along? In 'Contraptionology!', it's what if Applejack tried to join a science fair? 'Super Amazing Rocket Sled of Awesome' asks what if Spike gets a... well, you get the picture.
And thanks for the 'Changeling in Ponyville' recommendation. I'll be sure to check it out.