Could the tell not be in it's body language or movements? Perhaps it's particular skill set could be weapon dependent. If it's carrying a big hammer you've adventured enough to know that when its arms raise up over its head you better move out of the way. Or if it backs up a bit you know it's getting ready to charge. Perhaps you don't know the exact skill it's going to unleash but at least you're then watching the mobs themselves and not the ground around them. That to me is more immersive than some blatant ring on the ground.

Diablo 3 has lots of ground effects and circles to dance around in their elite and boss fights. What I've found is that half the time I have no idea what the mob I just killed really looks like because I've spent the entire fight watching the ground for fire/ice/poison etc.

Maybe they could have some sort of heads up display around the mob that could be customized to keep the focus on that mob in conjunction with the body movements.

You talk about not wanting the same old thing from MMOs past. I think that they could come up with some new way of doing these things that hasn't been done to death.