When you take a long look at World of Warcraft it's largely an amalgam of things EverQuest and subsequent copy-cats did a little bit differently and/or slightly better. The great Spanish painter Pablo Picasso said, "Good artists copy. Great artists steal." and the same is true of MMORPGs and games in general. The genius of Blizzard is that they know what to take and make their own and what to leave be, at least most of the time.
Age of Conan and Tabula Rasa
I think it's highly likely that Blizzard's next venture into MMOs will have a faster form of combat. Not only that, but the game will most likely exist within a setting far outside that of World of Warcraft's to avoid the cannibalization of that game's audience while still having plenty of room to grow another. It'll either be Starcraft, a new IP or a licensed IP. I'm betting on Starcraft, but there's always the off chance Blizzard plays it way risky.
With a Starcraft -- or any sci-fi setting, really -- MMO you see why I bring up Tabula Rasa. The game may be shutting down, but that's through no fault of the combat designers who worked on it. In fact, one of the things I really loved about Tabula Rasa when I played it was the way shooting was handled. Granted, it wasn't a fully non-stat system, but it was a great start for something even better. And who's the company that excels at taking a good idea and crystallizing it into a great one?
Terran troops shoot, Protoss troops slice and Zerg... well they do both. If anyone could marry and improve the best combat two games have to offer, it'd be Blizzard.
Here's an either/or:
Either Blizzard will make a Starcraft MMO that's all about ships in space with the best parts of EVE Online and some other games grafted on, or they'll do that in addition to what I just wrote about above -- and it'd be pure genius.
Oh, and here's an also:
If they're smart, they'll also figure out how to make EVE's skill-based progression system work for scaling PvE content. Not only that, but those wily developers should figure out how to retain the "DING!" effect without having levels in a game.
A skill-based leveling system is preferable in that it allows a rather young character to experience content with a significantly older one. However, the issue is that the carrot which MMOs use to keep players moving forward is drastically reduced when all you have to do is set a skill to learn and wait hours to months -- like in EVE.
To me, the big problem here is that MMOs lack an engaging moment-to-moment gameplay mechanic. They're all about playing with other people over long periods of time in a persistent world. While that experience can be amazing, it's also a hard pill to swallow when everything consists of "1, 2, 4, 3, 1, etc" combat. Does it need to be Street Fighter IV or God of War 3? Hell no, but obviously MMOs can afford to lean a little more in that direction.
This all goes back to the Age of Conan and Tabula Rasa combat. If Blizzard can nail moment-to-moment combat that holds up half as well as Call of Duty 4's in multiplayer, they'll have something potentially bigger than even World of Warcraft.
Finally we come to Guild Wars, whose subscription model will most definitely not be showing up in the next Blizzard MMO. What I think would be very smart, however, is to allow for streaming content as an optional feature. Some people have bandwidth caps on their internet connection, and prefer rather to download content only when and if they want it. So the smart thing to do would be to allow for either larger chunk updates, or streaming content.
Another feature Blizzard needs to ape from Guild Wars is allowing people to play with NPC classes in tow. This will both streamline their design and development pipeline by allowing them to always assume players will have a party of say, five, and give people a chance to learn how their role interacts with other roles early and often.
I highly doubt all of these things will make it, but you can bet at least a couple will find their way into the game. Or, even in the unlikely event that none of them do, other games' features will certainly be mirrored one way or another.
Ultimately, even a little bit of World of Warcraft's best bits will make it into the next game.
Whatever is being made in secrecy right now will probably surprise us in some ways while at the same time seem completely obvious after being revealed. It's a conundrum how they do it, but maybe that's part of their method to success.