Off the top of my head, one cool thing would be:
A horse riding system
The “horse” could be any interchangeable creature mesh, but something similar to Shadow of the Colossus where the player has control over the rider to do actions independent of the “driving” while on the horse (stand, shoot, lean sideways, etc.)
I would be big bucks for such system.
2nd: A bow and arrow system 
3rd: A climbing system similar to Shadow of the Colossus.
Many others, those are my first 3 quick ones as I’m browsing. Weather control and particle assets would be fantastic as well.
Ragdoll mixed with baked animations, not sure what to call it. I’ll get into that later.
The Market is gonna be explosive!