According to Russian datamining site, Horadric.ru, it appears that some new heroes and villains will be heading into Marvel Heroes Online in the near future. This is all speculation, of course, but the information seems strong. Use Google's Chrome browser to see the site translated to English.
Our own Mike Bitton offered a couple of thoughts:
These guys have been pretty good at datamining new costumes coming to the game (one of 'em is coming out today) and it appears they possibly have datamined out upcoming unannounced characters, including two villains (Juggernaut and Moonstone). The latter bit is particularly interesting since you can actually fight Juggernaut in the game and, you know, villains!
Check out the site above to see what all the rumor is about.