I think there was a misunderstanding. Sorry
I didn't mean Collide with like an animated action. I was referring to Character Collision.
What keeps characters from falling through the terrain or bumping into one another.
It doesn't seem very costly to me.

They had it in at one point, and complicated enemy AI, but they removed most of it likely do to server lag and targeting issues. To me that's no reason why they can't bring it back with the new servers. They can probably use most of what they had originally and just improve on that.