Mob agro from unseeable mobs?

1.0 already had this! It was called 'mobs near zone lines'. You were calmly traversing Thanalan and lo and be hold, what is this? Four L99 drakes feasting on your corpse!

On a more serious note, however, you're being a little shortsighted in thinking that this would work well in an MMO. MMOs are extremely dynamic in that everything is done in real-time and there is no transition between in combat and out of combat, random encounters don't fit well with this it'd be in the best of the days a jarring style-break and in the worst days really goddamn annoying.

Random encounters work better in a type of game where in combat and out of combat are very separated, like oldchool RPGs were when you don't need to react to a monster's presence or not in your surroundings, but to it's action once combat started. Not saying an MMO can't work like that, I dream of an MMO that works with turn-based line combat... A pipe dream to be sure, that is definitely not a thing that'll ever happen, but oh well.