This would be a good way to incorporate sustain for mages without it going crazy out of hand. The biggest issue I see right now is that AD champions can build items like Maw to avoid being bursted, and they can sustain the damage back with vampiric sceptre or doran's blade. This gives AP champions basically no window to kill them in a 1v1 scenario, and it's a big reason Zed is so toxic at the moment.
Giving mages a sustain option would be fair, considering they're already forced to manage their mana.
EDIT: Okay do any of you keep up with red posts? One of their priorities in the mage rework is to adjust Vlad so that spell vamp isn't just a niche pick ruined by one champion. Now is the time to be making suggestions like this because they are balancing for it as we speak.