Re: Please help me with new game setting
Perhaps there were no non-powered heroes. This seems suggested by your gene-bomb: there's a unified origin for 'supers' and it's genetics, mutation. No tech heroes, etc. A few costumed vigilantes here and there, sure, but once all the superpowered people go away, they'll be few and far between.
That said... once the super-powered heroes are gone, why wouldn't there be batman-type vigilantes in the meantime, people trying to "fill the gap" even though there's no way they could, since they have no powers. Second-raters, etc.
These guys might be pretty ticked when the powers (and the superpowered people) come back, since in the 20 intervening years, they'd be the ***** of the walk, so to speak. There's nothing I can think of to stop people from trying to be heroes. Get rid of the powers, and you'll probably still have costumed vigilantes Batman-style (even if the original ones are all killed in battle, people are going to keep trying). Is there any particular reason you need them gone too?