Somewhere in World of Warcraft and other MMORPGs is a random number generator. Your fireball might do between 157 and 218 points of damage, with a 5% chance of a critical hit for 297 points, and the random number generator determines the outcome. The problem of that is that the player doesn't care too much. On average the above mentioned fireball does 193 points of damage, and that is all that counts, because you use the spell often enough. The random number generator only kicks in *after* you made your decision to cast the fireball, so the random event has no influence on your decision. To make random events more interesting, they need to happen *before* the decision process.
I just finished Metal Gear Acid 2 on my PSP, and that is a typical example on how to use random elements to make tactical combat more interesting. What "cards" you have in your hand is determined by the random draw that happens before you act. You can have 30 to 40 different cards in your deck, but your actions in every round are limited by the random 6 cards you are holding in your hand. Your decision depends on the random draw, because you can't use the rocket launcher if you didn't draw it. Thus even if you repeat the same combat several times with the same deck, it will always be different, because you draw the cards in a different order every time. That increases replayability a lot.
So why aren't there any MMORPGs with random elements influencing your decisions? In WoW you quickly find what order of spells works well and then usually use that same order over and over for hours, making every combat nearly identical. Chronicles of Spellborn announced they will be using a skill system in which you won't have the same hotkeys available every round. But there is no randomness involved, you manually put the abilities into hotkey bars 1 to 6, and they just rotate through that sequence.
Having a "deck" of random abilities from which you draw is one good system for introducing random elements to which the player has to react. But it isn't the only one possible. One curiously under-used way is random behavior of monsters. Ever seen a mob raising its shield for some time, making sword blows less effectice against it, but opening up a vulnerability against kicks? A few boss mobs in WoW have damage shields or magic shields, to which the players have to react. But in the standard solo PvE combat the mobs rarely do things that would force a player to react. If a mob has resistances, it is always the same, like a fire elemental being resistant to fire spells. There are no mobs with random resistances, which you only find out after the first spell you use on them. If monsters would do actions which make certain player abilities more or less useful, that would make combat a lot more interactive.
More interactive combat would be easy enough to implement in classical MMORPGs. The trading card style of combat from Metal Gear Acid would need a completely new style of MMORPG. Game companies are often highly conservative, rarely daring to introduce radical innovation. But the potential spoils could be huge. Because once you have trading cards as the base of your fantasy online world combat, you have the possibility to sell random packs of cards. Of all the games I played in my life, I spent the most money on Magic the Gathering, by a large margin. The urge to buy more cards to make yourself stronger, or to complete your collection, is strong. I'm really surprised that nobody has tapped into that urge for online roleplaying games yet. I've published the idea years ago, but nobody wants to borrow it. A "free to download - free to play - pay for added cards" model could be very popular, as it enables everybody to play according to his financial means. That would be a welcome change from the current games, where only your disposable free time counts.