Ideally an MMO IP would have an interesting and well-established world with some mainstream popularity. The world has to have some initial draw, being somewhere that people would be interested in running around in and exploring. Another requirement would be the potential for interesting classes, and all the better if they are not simply rehashes of other game classes. If all you have to offer is a cliched Ranger or Wizard or Pirate, most MMO veterans would yawn and pass. Not that it's impossible to do something intriguing with some well-trodden roles, but you have to take into account how jaded many of us are.
Of those IPs mentioned, I see Firefly as a good SF candidate. It could combine elements of games like Elite and Freelancer with more common MMO aspects. The trick would be fleshing out a good class system, so that it wouldn't just be about moving up to bigger ships or be too EVE-cloney. You would need to incorporate trading and combat and things like espionage and diplomacy.
For fantasy IPs, GoT is a likely option, as is WoD. Game of Thrones is a relatively low-magic world overall, so that might be tricky for players who want a mage-type character. It might also be harder to keep interest if your character is just a rank-and-file pawn, given that the well-known characters would have to be strictly NPCs. Leveling up would have to be more than just commanding more troops, unless the intent was to work in RTS elements. World of Darkness holds a fascination for many, and has a well-fleshed-out tone and lore. The clans provide ample class-building room, and the inherent magic allows broader skills than just claws and weapons.