How about exchanging Ultimate Hat with Overheal?

Let's face it: nimbus cloak is horrendous for the vast majority of people (mages) who take sorcery as their main tree. It may be a good option for champions who looks for mobility in a second tree, but already more than half of Sorcery is related to mobility, so...

Mages have been asking for a way to sustain since spell vamp has been killed, and this is the perfect opportunity to have one: bring overheal to sorcery (which was already underused in precision compaired to triumph) and rework it in Overcast:

Overcast: The first spellcast who hits an enemy champion generates a small shield who's size scales with the magic damage delt with the spell.Each subsequent spellcasts who hits an unit for a short duration slowly converts said shield into health.

Your thoughts?