One I haven’t had a chance to run yet, for an all mindflayer campaign. Mash together x-com’s council person and half-life grey man as your boss, and make the PC’s the baddies (sorta).
PCs were modern day-ish operatives who got kicked through a dimensional gate, get stuck in mindflayer bodies, and have to do spooooky stuff in D&D universe to protect/heal their home universe from getting wrecked by another group of mysterious invaders.