Well, basically there should be things like a main earth faction but not everyone's united and you get these little sub-factions with their own interests and whatnot. Then you have like aliens plotting behind the scenes as well. You'd also have outright supernatural shit like demons, dragons, spirits and whatnot powering mechs or them being mechs in and of themselves. You'd have over the top hot blooded heroes to go along with the more cool rational ones. 

In short, it'd be a setting with a little bit of everything.

[url=http://en.wikipedia.org/wiki/Super_Robot_Taisen:_Original_Generation]Just to give you an idea[/url]