Disclaimer: I'm talking about designing a system around PvP combat. That combined with good AI = much deeper PvE experience than most games, so bear with me even if you don't like PvP...
Standard MMO combat doesn't require much thought. Oh, sure, you've got to spend several hours researching your build to come up with the most leetest character with which to pwn all, and you've got to grind for a few hundred hours to get the right gear, but once all that's done, a monkey could pretty much be playing your character.
Depending on the MMO and your class, you'll either spend most of your time auto-attacking, or pushing the same couple of buttons over and over again.
I'm hearing more and more people say that they're over this kind of system, they want something deeper. And game developers are apparently listening, for example one of AoC's biggest selling points is its 'skill based combat' involving a bunch of combos that have to be executed by the player. This is a nice idea in theory, but it sounds like it's going to require twitch reflexes and heavily favour whoever has the slightly better ping.... I'll play an FPS for that thanks.
Another way for a combat system to go might be one that is less intensive on reflexes but more intensive on micro-management. Think Starcraft in MMO form (somehow). This again doesn't appeal to me, as I don't have crazy korean micro hax :)
That leaves us with one more option - combat via some kind of turn-based strategy system. Goodbye lag issues, goodbye micromanagement, hello tactics.
One blogger I read recently suggested a system similar to Magic : The Gathering. This is a nice, balanced game (ie many different builds all fairly much equally viable in the right hands) with loads of strategy. However 2 things that don't really make it suitable: 1) The luck factor. If you don't draw the right cards and your opponent does, there's often nothing you can do to win ; 2) The game is based around summoning creatures that attack and defend for you. I can't see the majority of players wanting a system like that, they want to feel like THEY are the uber leet ones, not their pets.
Another game worth talking about is Guild Wars. True, Guild Wars does revolve around the standard concept of pushing a few buttons over and over to fire off your skills. But thanks to the way the different classes were designed, there's a very rich interplay between them all, and various tactics are made possible by virtue of map design. The only problem is, the game seems to be designed around one build, the so-called "balanced build" (for those that never played guild wars, it was 2 melee, 2 anti-caster, 1 support caster, 2 healers, and a flag runner). This led to cries of a stale metagame. Any deviation away from this metagame was usually nerfed (as the deviation would otherwise just become the best build). So top end GvG often looked pretty damn boring.
The next big thing in MMO's is going to need to have multiple builds all equally viable, all with equal chances of beating each other, and all feeling completely different to play compared to each other - ie the choice of build should be purely an aesthetic one that is made before the match starts, and has no impact on your odds of winning that match. Each build should have multiple tactical options open to it. The more builds that are viable, the more interesting the game feels to play, and the longer it takes players to get bored with it. Looking at Starcraft again - a beautifully balanced game, still I believe one of if not THE top game in Korea. Number of aesthetically different options available to player before the match begins? Three! (there are 3 races to choose from, all equal, that all feel extremely different).
It turns out that it's pretty damn hard to design an RPG with a turn-based combat system AND lots of different yet equally viable builds available. But I do think it's possible, and have a lot of random thoughts floating around on the subject which I shall start going through next time. Feel free to share your own as well! Who knows, maybe somebody will listen...