Player perspective (3rd person, 1st person, etc)
This would be a 3D game right? I think this would be a no-no cause not everyone has a 3D engine.

Interactive 3rd party characters / Storytelling / puzzles & problem solving
Sounds interesting

Item gathering
Yeah, you could make a RTS with it (gather resources, build tank or whatever)

My idea was some sort of race. Start from point A and be the first one the reatch point B (or goal B).