How about a system where you spent a feat, chose a spell and you now cast that spell better. The spell itself doesn't take up any more resource than before, but you had to spend a whole feat for a single spell improvement. So a real specialist might dump 5 feats into fireball and cast the best fireballs ever, but another wizard might spend their 5 feats to make slight improvements that affect all their spells, like spell focus and spell penetration did for 3E.