for the few mage characters i had, i used supreme magicka and modified it to take a bit less than two minutes to completely refill the bar. this made a huge improvement in gameplay. i had to carry around magicka and health restore potions for a change, previously i never bothered with them cause you never had to use them. they both became very useful after that and i cant tell you how much more fun it became. it didnt hurt my spellcasters in any definable way it just meant i had to play smarter. i made sure i was stocked up with potions ahead of time and yes there were times when i had to retreat.........which is a GOOD thing in a game. always pwning everything all the time gets boring real fast.
I agree that something like this would improve the game. When you read fantasy novels, or literature on similar topics, the spellcasters aren't going around throwing spells left and right. Usually what they do is used in key situations, and afterword they are usually tired.
Personally, I think that magicka should regenerate slower, but I also think that you should be able to cast spells when you're low- so long as doing so taps directly into your HEALTH or ENDURANCE when your magicka is too low. Like debit card, sort of. Just like in fantasy literature when an exhausted wizard digs too deeply to cast a spell it has adverse physical affects on him. And of course, such things would raise the value of enchanted items to help with magicka- IN FACT, that seems to be what wizard staffs are used for in fantasy literature, to AMPLIFY the wizard's ability to cast magic without draining himself too much.
Otherwise, you just get staffs used like random tools, carrying 50 of them and using each one depending on which spell you needed (very much not cool, IMHO).