One of the things I disliked about D&D 3.x was the fact that you had to level up to get the next coolest thing, especially if you were a magic user. I find that boring and tedious. If I'm a magic user I should have access to all the spells/powers available to me and not be restricted by some invisible barrier.
A consequence of this leveling up process is that a magic user at 10th level is really no different than a magic user at 1st level. The spells aren't harder to cast, they don't require more from the magic user, they simply do more damage. Accepting this as a basic premise, why not make all spells available at the start and then adjust how they work?
With that in mind here are some thoughts that I had about how to let magic users have access to everything at first level, but not make them overpowered, or boring because there's no challenge.
--change the damage done by a spell. As a magic user increases in power and knowledge his spells also increase. So he might start out as a d4, then move to a d6, then d8, d10, etc.
--change how many spells per day the magic user can do. Maybe at the beginning he can only do two spells per day and he's worn out and drained. Later on he can do four.
--increase the chance of failure at lower levels. If the casting of a spell had a difficulty of 15 normally, maybe a 1st level user could only cast it by hitting 18.
I also like the idea of a lower level magic user still being able to cast a higher level spell, just making it more difficult to do and more draining once it's been done. Similar to someone drawing on those last reserves of strength and will to accomplish things and get them done.
On G+ Ken Austin had a great solution for Pathfinder, where the magic user could take a feat of something like "Over Achieving Spellcaster". The magic user could cast a spell at any level, but would have to make a Fortitude save of 15 plus the spell level, plus the damage done by the spell or remain unconscious for the number of rounds equal to the damage done. This would allow a player to sacrifice himself for the good of the party in a grand heroic gesture.