-Try and keep your ideas simple and realistic, no game changers.
-Try to be unique and stay away from the hot topics too (like dragon riding).
I'll start it off:
1) Bar tab
-You go into an inn and can eat and drink whatever you want on the tables or otherwise, and its only stealing if you run away without paying. Additionally, you can bargain your price at the end with the innkeeper.
2) Shield on back
-Why not? Looks badass.
3) Diving
-If you jump into the water, its the normal jump animation and you land floating/treading water. However, if you run and hold the JUMP button you got into a diving animation and end up under the water swimming. Easy to do in first person (with arms out), and could make for some cool diving spots.
4) Vaulting
-Instead of jumping over everthing, you go into a "vault" animation where you put a hand down to help you over.
5) Climbing
-Climbing ladders, vines, etc.
Post some more good ideas, or add onto others!