I think those thinking that democracy or any government would collapse are way off base. Huge armies exist today with weapons of mass destruction and are held in check by other armies with weapons of mass destruction.(Super simplified yes I know) The same would hold true in a world populated with super villians and super heroes. They would hold each other in check. Sure you have a Doom in Latveria but you would have a SHIELD or Checkmate to keep tabs on them and keep them in line.
More worrisome is the introduction of super technologies into the economic structure of the planet. Unstable molecules would severly alter the clothing industry. Teleportation would ruin the shipping industry. There would need to be a ban on laser weapons. Gene altering chemicles would need to be kept in lockdown least the entire population mutate.