I remember when fighting a single mob solo, even equal level, was a risky venture in most MMO's.. But I'd sure hate to see some cheap way out by just increasing attack, defense and hp.

More mob AI would be nice. Fights are rather predictable. Any mob w/ a special move.. well, rest assured, the moment you use a TP ability, guess what's coming next...

Or the horribly predictable "attempt to escape" you get from the last mob in a levequest.

Add a mechanic where mobs go for, or call for, reinforcements. Give them more branching options in combat mechanics rather than an extremely predictable chain of events that leads to their ultimate demise.

I remember the first time a mob tried to run away from me in WoW, I was actually shocked (my first MMO was XI, second was WoW)... Now here we are 7 years later, and newer games like XIV, and even SWTOR have seemingly taken a step or two backwards in this area.

Mobs don't have to be beefier to pose a challange, they just need to be a little smarter.