Sure, there are certain games that would easily translate into an MMO, but there are also plenty of microcosm experiences that are fun, compelling and interesting simply because they're offline-only. One such example is The Elder Scrolls series -- or free-roaming-worlds if you like as Fallout 3 is now a part of these games -- simply because what makes games of this variety so much fun is how the entire game, its world and its people, react to you as a player. This sort of game mechanic just doesn't carry over into the online space.
Developers could certainly make a ghost-like facsimile of a mechanic where NPCs might react differently to you depending on what you've said or done, but that's missing the bigger picture. Features that are usually dependent on a single player can be shoved into a different genre, but why? It's just not possible to take a game where the coolest features are entirely based on a single person's actions and translate that into a game where 5,000+ people are involved.
For instance, in Fallout 3, players have the opportunity to decide whether or not to set off a nuclear bomb in the town of Megaton. You alone get to make that choice and it will literally change the entire landscape of the game-world, resulting in a very different story than the one a player who makes the opposite choice gets. Features like that are why games like Oblivion and Fallout 3 are so much fun and It can't be replicated properly in an MMO. Instead what you'll get is a quasi-similar feature that really isn't all that different from Everquest or World of Warcraft which ends up being kind of useless. MMOs are primarily about a community-driven experience, whether that experience is about getting the best loot or having the most PvP kills.
The truth hurts, but some games just shouldn't be made into MMOs. There are several titles to be argued for like Spore (which is very similar already) or Civilization, but there are also games that really work best as an offline experience like The Sims (For those of you who tried it -- did The Sims really translate well to an MMO? Subscription rates suggest not) or Zelda. Then again, many people thought taking a game series like Warcraft and making it into an MMO wasn't going to work. I'm always up for some surprises and have nothing against being proven wrong -- so long as the end result is good. You can turn The Elder Scrolls series into an MMO, but turning it into one that retains the charm of the original games is the real challenge.