(Warning: This is a bit of a rehash of an idea I had a long time ago... still worth mentioning and posting here I think)
In almost every player's MMO life, in every game, there comes a time where combat becomes a rote exercise. Ok... group of three guys, start with the debuffs, then my AoE attack, then pick off the weak guy, if I get into trouble, try my hold and retreat... Teams are often even worse. Tank goes in and grabs aggro. Healer heals the tank. Cannons assist the tank and wait until he has sufficiently taunted the spawn. Crowd Control be ready for adds...
Now, there are slight variations game to game, and I still think that in the right situation, any game can produce a challenging and fun combat. It is just that after a certain point, the cool combats become few and the mind-numbing, "hit the 2 button, then the 3 button, then the 2 button, then the 4 button..." stuff becomes more common.
Often, I will hear talk, or read posts where people talk about "better AI" in a computer game. In fact, one of my friends was recently bemoaning the boring combat AI in CoH. "I feel that combats in CoH have become dull and repetitive, Chris. Certainly a better AI would be the proper solution." Well, he didn't really say it like that. In fact he was a tad drunk and so his critique came out more like, "Dude, the f'ing opponents in this game are f'ing stupid. Those f'ing programmers need to stop f'ing nerfing f'ing Energy Melee and write some f'ing AI!!! (burp)."
But here's the key: programming a computer opponent for a game is hard. Programming a computer opponent that can actually engage and challenge a human player with any level of skill is even harder. Chess programs are perhaps the most successful examples of this type of program and the techniques used there, not to mention the coding effort, and processing power involved would not translate well to an MMO.
The good news is that you don't really need a good computer opponent to run your MMO combats. You are trying to provide interesting and challenging combats; you aren't actually trying to win against the players. Players don't want to be beaten as much as they want to be surprised, challenged and given interesting situations to which they have to react. The current aggro-based system in CoH does none of this.
What if we change the system and give the computer opponents a few more behaviors from which to choose. When the players first engage a spawn, it usually behaves in the old, dull way. However, there is a small chance that they will react with a pre-set behavior, chosen randomly from a list (but based on the characters in the attacking group). In CoX, I can imagine that if a group had a boss in it, the boss might call out the orders to his minions, thus giving the players a heads-up as to what was coming.
Divide and Conquer -- All enemies in the spawn pick a random target from the player team and aggro onto him. Player tanks and crowd control will have to act quickly to save their less hardy teammates. The aggro is enough that a single taunt won't cause the enemy to retarget, but it will take multiple taunt-type powers, or holds to get the situation back under control.
Scatter -- The enemies aggro as normal, but also do their best to keep players at a distance and use ranged attacks. This spreads the fight out, making AoE attacks less useful. It also adds potential danger as the PCs spread themselves out to engage the outliers, possibly getting into 1-on-1 fights they cannot win, or wandering into other spawns. A disciplined team ought to be able to stand their ground and pick off opponents, but an undisciplined team might get themselves into trouble.
Spring the Trap -- The enemies aggro as normal, but also cause a similar-sized group to spawn nearby and rush to the combat. Within a few seconds, the new combatants arrive to make life difficult. Crowd control characters will have to pay close attention, find the new spawn and neuter it before any damage can be done. Alternately, for a nastier surprise, the enemies can teleport in, or deactivate their invisibility shields and appear right on top of the fight.
Use the New <blank> Rays We've Acquired -- The enemies aggro like normal, but instead of their normal attacks, they get a new attack of a random type that they don't normally have. Not a big deal in most cases, but it is possible that the new attack type hits a specific hole in the characters' defenses (could be psi in CoH). Players will have to grin and bear the new attack type, or use holds to prevent from being attacked.
Run for Help -- The enemies flee from combat to the nearest spawn. Once they reach that spawn, both spawns will turn toward the players and attack. Players will have to kill the enemies quickly, slow them, or hold them to avoid adds.
Bob, You Run for Help -- Similar to the above order, but instead, only one guy will run for help, while the rest of the spawn fights. If Bob reaches a spawn, it aggros to the players' location and Bob will continue to the next nearest spawn. Players will need to find Bob in the chaos of battle and deal with him quickly, or else the whole map could come down on them.
... and so on... I could think of more, but I think you get the idea. What do you think?