Trash already behaves this way for many bosses, but I feel like it used to play a bigger role than it does now. Haven't done Normal+ raiding since Wrath though. Trying to remember why specifically I seem to recall trash behaving more like this in the past than presently but drawing blanks except for Molten Core.
They need to do more of this and less "waves of trash" like ToT.
They've been doing it for awhile now example the Dumuru trash =)
Yup, there's a lot of boss fights where the trash has abilities the boss uses, or similar ones.