There's a difference between "silly" and "stupid", and you really shouldn't get those two mixed up.
Edit: Of course, we could start 'later' as a big starting event is happening like the alien invasion thing you mentioned. Of course, I'd personally have a more 'down-to-earth but not really' beginning arc. For example, I'd prefer if a 'doctor doom' or 'lex-luthorish' sort of figure was making a power-grab in the upper echelons of society and the PC's needed to stop him before he went totally dictator, but hey whatever.
IF the cast of characters/players is large enough we could possibly have maybe two cities side by side. One would similar to Metropolis and the other like Gotham.
<Snipped quote by The Kid Lantern>
See, I kind of find casual murder and overly cynical/gritty attitudes in my heroes a little bit boring if it's a widespread thing, personally. Like for example The Judge character in this RP kind of seems to lose his luster and point if all the other characters are also super-violent or anti-heroic
Isn't the golden rule of comics that when two heroes meet they must fight?