Hmm, I would have to an Animals Update. More accessories and availability for mobs, both passive and aggressive(and neutral) mobs. Like wolf armor or styles; Enderman variants(may require or suggest an End update to the main island); more varieties and uses for cows and pigs; re-adding the killer rabbit?; including add ons or special variations that make mobs more dangerous, like burned zombies(stronger damage and immune to sun), marrow skeleton(more health and shoots bow faster), Sentient Enderman(bigger, more aggressive. Maybe a sort of mini-boss?); we are getting the Warden in 1.17 so I'm curious as to what that'll bring. But other than that, I think maybe they could revamp how certain biomes work and their generation. Maybe make new biomes and re-add Jungle Temples(I don't know if they're removed or if they're just so freaking rare)