Rechargeable fusion bolt would be rediculously broken, because you would just be able to do the fully charged ice/fire fusion with or the ice/lighting one that hits 5 targets repeatably.
I wouldn't say it's any more broken than Windmill hahaha. You see, if something could be changed for mages that would release them from being so outclassed, Fusion Bolt's one of them. And the damage that it CAN do doesn't compare to a windmiller at the same level range until you have two of the bolts you are using ranked and a ton of int. I'm still outclassed too and I have like 500 int usually.
If you can cast a full charge of Fusion Bolt and do 2k crit damage every time (really REALLY want this to be true) - with proof - then I can rest. However, that doesn't change the fact that my fingers never rest when I'm using Fusion. Fusion all day every day.
Then again, I still expect you to have used enchants and ranked many other skills to increase your chances for Fusion to be viable. Unfortunately am pure mage so I'm a little out of options.
Edit TL;DR It would be a great feature. Not broken