I built a homebrew mage for 3rd edition that started with 3 spell slots and escalated to 12 at 20th level. He had a limited number of spells known (his intelligence modifer I think) that he could cast spontaneously. The spells were seperated into 4 tiers. Many of the lowest tier ones were built to be useful multiple times throughout the day, kinda like encounter or at-will. He gained the ability to learn metamagic style abilities as he gained levels. These could instead be traded in for more spells known or more spell slots per day. Also, whenever he gained a level he could exchange any spell he knew for any other spell he could cast. The key was in the spell slots and limited spell selection. There were no "levels" to the spell slots. You could use a slot to cast any spell you knew. With the limited spell selection sometimes a lower tier spell would be much more appropriate than your strongest spell, and they saw use. Also, the spells were (supposed to be) designed to scale well so even the spells you had at 1st level would remain useful into mid and possibly even high levels. The limited spell selection also made each character feel unique. It never got past a Beta stage, but everyone I know who saw it greatly prefered it to the 3.0 wizard. Looking back it could have used A LOT of improvement, but it had potential. Of course that wouldn't satisfy enough people as the only sort of caster, but I'd like to see something like that in 5E.