I would run a Xenogears-esque mecha & superpowers game, except with a steampunk/WW1 vibe, and I'd run it with HERO. It would have a military theme (large scale combat FTW!) involving dozens of mecha and characters capable of fighting them on foot in many encounters. Sometimes the PCs would fight in-mecha, sometimes out. In some adventures, they would switch from mecha to on foot several times during the course of the scenario, regularly splitting the party as one character infiltrated while others fended off opposing mechaneers, etc.
Frankly, if I didn't have to worry about generating stats and had a computer with a program capable of doing all the numbers work during play, I'd use HERO for everything.
As it stands, I mostly use Mutants & Masterminds for everything, but that particular concept really pushes the envelope of playability even with M&M.