So Aggro, enmity, hate etc different game, different name now I've only played on FFXIV as my MMORPG but they way they handle the Aggro system is based on how much damage you do and some skills have a aggro multiplier to it but when it comes down to it the more damage you do the more Aggro you generate.
Which is all fine and dandy like but for new players playing with pro players this could prove to be a issue as a DPS could easily rip Aggro off a newbie tank due to no fault of the tank trying his best.
So currently thinking of my MMORPG I've been thinking of implanting a Aggro system but give skills base Aggro values instead of it being determined by damage amount. So in this case even when say a Pro Mage is dealing HUGE amount of damage a newbie tank should still be able to keep Aggro just fine.
You could also do it based off of the first player that the monster targets, which could be both bad and good, since adds could pose more of a threat, where as many bosses couldn't aggro anyone other than a tank.
First in = Aggro for the fight, any ranged classes would need to kite if you are going to allow spells to be cast while moving or if you want to set up instant cast spells.