Honestly, I want a superman game that takes 3-4 years to develop, and lets McFace it, that's not going to happen.
If such a game were to exist, it would need to have either free-roam or very large levels to accommodate Superman's extreme mobility. For those who think of Free Roam as a map with little repetitive missions on it, think of it as a group of large spatially non-linear levels all adjacent to one another.
That said, having just a Metropolis area gets old quick, and even that free roam doesn't really accommodate Superman's mobility, y'know? You gotta give the player the option to go to space, or Smallville or the Fortress of Solitdue, even if its not continuous.
Perhaps the greatest challenge of Superman is making him feel epically heroic while constantly challenging him. I don't think the mobs-in-the-streets format really works for Superman, because there shouldn't be armies of guys that can challenge him on a regular basis. I think the Shadow of the Colossus method may work better: giving Superman about 20 truly epic boss battles, maybe a couple that are 'mobs in the streets' events, but honestly... thugs? Superman? Yeeeeeah.
And then the thing that would make the game actually worth talking about, the little touches. We need some Clark Kent in there, not full on missions really, but little extra objectives here and there, mostly for story purposes, but they're useful for setting scale. And of course, the set pieces and environmental interaction to make the game the exhilarating fun spectacle it needs to be.
So, yeah. We'll see. SR took several steps in the right direction, the rest of the steps need investment from the publishers.