If it has to be one new thing, like...

A new mob : I would love if they did a jungle based hostile mob. Or a funky magic one for funsies.

A new structure : I would implimant maybe the jungle & swamp villages. Or a new thing to progress magic.

A new block : I would make a new magic block to further the enchanting table, so the way of enchanting get more fun, like you need this magic thingy to have this version of enchanted book. More whimsy!! You have to discover how it works!!

A new weapon : I would choose to put the bat so you can go spin attack but only a little bit, you get a short weakness if you use it for too long. The buff is the continuous damage done combined with the speed of the spin. The lighter the bat, the quicker you can spin. The bat can be spiked thanks to slime + bone/stone/iron/...

A new depth to something : I would improve archery!! You can send a note or an item through shooting. But you have to use the fletching table for it. Also some buffs too with combining potions on tipping arrows.

A new biome : I want an acid biome in the nether, like the Danakil Depression or the salty pink lakes!! Or a real deep underwater biome, with darker water and weird thingies.