In vanilla Minecraft, Withers can shoot wither skulls. I would like it if that was an action that applied to Wither morphs. Basically, by being morphed as a Wither, you can shoot Wither Skulls by pressing your action key, the type of which is determined by whether you are holding Ctrl (blue) or not holding Ctrl (black).

Additionally, it would be nice if Wither morphs healed 5 HP for every living mob kill they made, since that is also vanilla behavior for Withers.

Note: Neither of the above ideas are currently implemented.

Multiple abilities is a can of worms and needs some design consideration; modifier keys don't scale well to an arbitrary number of abilities.

The Wither does need to be improved at some point, not sure when.