When it comes to popularity, image and reputation go a long way. The world of video games is no different. A good character can open the door to a variety of new possibilities when it comes to a game franchise. At some point you would think that a company would use characters with such popularity to get involved with all forms of gaming.
The reason fighting games are so popular are not only due to their classic arcade-style game play but largely because of their abundant character selection. This is the biggest reason I see in taking a mass multiplayer endeavor. Once again, a DC Universe or Final Fantasy Online style game with tons of well-known characters has potential. The MMORPG would obviously lose the arcade-style fighting that help these games to succeed, but still flaunt a huge selection of characters to fight and possible play as.