You can use (instead of roar) item damage bonus and item armor bonus, just make them unit abilities.
You use the transform (just like pharaoh said), and trigger in the damage and armor bonus. when the ability wears off, trigger the bonuses out. it will work, only downside is 2 ability slots are used. You can, however, set the icons and tooltips of the bonus abilities to null or none. also, If you want to mimic the animation of avatar, you can loop a trigger to increase scaling value slightly every .03 seconds until it is as big as you want it. Combine that with the avatar special effect, and most wouldnt notice the difference.