1. A way to place monsters
3. Flying capability
4. A way to adjust and or customize control scheme
5. Seeds (map ID numbers) and generated structures
those are the main ideas which I believe are realistic. I'll cheat a little and add more than 5.... Sorry.
Some unrealistic include:
6.rope with physics
7.bow and arrow
8. Bow and arrow with rope attached.
greatest of all would be
9. Access to the coding of the game so you could see how things work and possibly modify the physics and textures of the game.
10. Texture packs!