Warhammer online did this.

It's a mixed bag of useful and pain-in-the-!@#^. On one hand, woot, damage abilities go to the enemy and healsies to the friend!

On the other, self cast while targeting an enemy often saw your quick save-my-own-ass heal going to the friend. It saves a step in some places and adds it to another. Mouseover targeting is indeed the best way to handle this.

Also maybe improved and efficient interfaces like Wow had with GRID. Amg GRID. Square, pls.