So the reasons we did this:
We noticed that the players knew too many skills too fast, which made the game really easy.
Someone has already posted here that the current system does not limit you very much in the number of skills you can learn (put 1 point in 5 schools and you can now learn 15 spells at level 5).
What about basing the number of skills/spells a character can learn on his level? So, a character at levels 1 and 2, can know altogether only 5 spells, a character at levels 3 and 4, only 6, at levels 5 and 6 only 7 and so on. With such a system you would have control on how many spells and skills a character can know at a certain level...
Then, in order to make the character progression last until the very late endgame, you could increase the number of skill points needed to acquire the skill levels. Right now it is 1+2+3+4+5, isn't it? What about making it something like 1+2+4+8+12? So, a fully specialized character would be only possible only very late in the game.
And finally, make the spells again dependent on you specialization level and also on primary stats. So, if you say have something like 3 points in fire specialist and need to acquire 8 ability points to get level 4 in fire specialist, then you can still progress your character in-between since you gain intelligence and can learn level 3 fire spells which have a high intelligence requirement.
