This is actually a pretty neat idea, particularly since the mechanics correspond fabulously with SolarBeam.
So, some more ideas for it to throw out there:
Obviously, if it were to be based upon SolarBeam, it'd need some major damage fall-off over distance, so it's still only really useful as a close range attack.
It could be made so that it can only be charged while on the ground, with no partial charges, and possibly even without the ability to cancel it. Thus, Ivysaur would still be highly reliant on the tether, retaining that bit of his playstyle, due to not wanting to waste it. It'd be helpful when out of tether range, but the player must be wary of wasting it since you get only one per recovery (and you probably wouldn't have a new one by next time). It'd also be a difficult decision between saving it for when it's needed, and using it now to kill your opponent, with the ever-present threat of whiffing. Overall, this'd make it more akin to Wario's Waft than Diddy's Rocketbarrel.
Another possibility would to use it for the solo Ivy's Down-B.