Oh and the ideas for an open world mech/gundam things are great!
I always thought that on mech games, you could have several classes of weaponry:
Powered armor (The Size of the Power armor from Fallout or Armor by John Stakely...about 3 meters tall. propelled by feet.)
exo walker (about the size of the Titan suit from Jak 2, the EXO from Killzone 2, or the lifting suit from Aliens propelled by feet, or maybe even wheels, or hover pieces.)
Crew Mech (about the Size of Metal Gear REX, can be propelled by feet, treads, screws, hover pieces, or even biological components.)
Heavy Mech (Gundam sized. can be propeled by any of the above)
and heck there could even be some ships too. ship combat against Gundams could be awesome.