The classic setting for a mecha pilot roleplaying game, is likely BattleTech - or more specifically MechWarrior, the roleplaying part of the setting. It details a setting, of course, but also goes into some detail about missions that one can solve.

You may want to introduce professional rivals and so on, in order to up the ante somewhat for your players and their characters. In particular, it may be fun if there's some gloryhogging going on - that's sure to rise some hackles.

Depending on the setting, you may also have missions to retrieve artifact tech from an opposing side - uncovering atrocities during the mission, for instance. If you can get setting books for something like CthulhuTech, or a variety of GURPS books - such as Star Mercs or Mecha! - you can also mine these sources for inspiration.

The enemy is important in a setting like this - are they eldritch things from outer space (like Neon Genesis Evangelion or worse), are they giants (like Zentraedi of Macross), are they humans (from the future, the past, what), or perhaps fantasy monsters (like Gate: Jieitai Kano Chi nite, Kaku Tatakaeri)? All of this sets the tone for the rest of the game.