Here's the idea:
Lets start off with an MMO. Lets give it the regular things like leveling, quests, grind, crafting, housing, etc. Pretty basic, the exact designs can be anything really.
Next, incorporate other games into the MMO.
So, while in game, you can chat, run your avatar around, do quests, level up, etc. What if you are bored that day? Well, some MMO's have come up with solutions. Add in puzzle games to play! Like parlour games, bejeweled types, tetris types, etc. But wait, that's what MMO's do already. So lets take it a step further.
Make the MMO game itself a platform for launching other programs IN GAME, so that the MMO will track data from that game for rankings in the MMO in different areas!
Lets go with this: Say that Valve Software makes this MMO. when you get into game, you can run around and do quests and such, battle, form parties and complete story line stuff. But wait, there is a timed instance that you are waiting for! what does the group do? Go back to your house, then launch Team Fortress 2 and the party can join servers together to play games.
In the MMO's, you could set up tournaments, make bets for teams in organized matches, anything and everything. Don't feel like TF2? Why not DOTA 2? Take your 5 member party that is waiting for that instance to be ready again, and play a game of DOTA 2.
Only have a few minutes time between areas? Jump into a match of Counter Strike, Global Offensive for a few minutes while waiting!
I've always been a firm believer in the idea that MMO's are just glorified chat boxes. Mix it with a large content distributor like Valve and the fact that some games are free (TF2, and they gave out plenty of keys for DOTA 2, it might as well be free), and you've got yourself the ultimate platform to join an MMO to. It would appeal to casual and hardcore players. Even having the games available through a global menu like Global Agenda would work.
If you think about it, these types of things are already being put into place in many MMO's. Puzzle Pirates is full of games and such for completing tasks. Global Agenda has a very large MMO styled world to explore (though it's horrible) while waiting in ques for matches. Why not have a game where you can be burning people alive in TF2 while searching for a party?