Why wouldn't the elder scrolls system work? I don't understand. Ray of Frost should get stronger as I use it, just as if I were using a bow in Skyrim for example. At certain lvls of improvement I would get runes for it. I'd get lvls from killing mobs which would "gate" the level at which I could push my Ray Of Frost. It makes no god damn sense from either a player perspective or even thematically that I should get runes for abilities I don't touch. Why would missiles get better if I never practised it as a Wizard?
Look I'm not saying it should copy that system exactly but any system where the player is REWARDED for picking an ability is vastly superior to either the d2 or d3 system. I don't expect Blizzard to reinvent the wheel I just expected them to push the game forward a bit more. They did with starcraft 2 IMO and they've generally done that with wow expansions. They haven't in d3 IMO.