The other unique thing Ryze has that I think can and should be emphasized more is his mana scaling. They should roll with him being the most wizardy mage warlock magician in the game.
Maybe they could go all out and make him have unique scaling with all of the spell caster stats, maybe he could gain AD equal to a portion of his AP, health equal to a portion of his mana, and health regen equal to a portion of his mana regen?
Maybe make him have very poor AP scalings but increase his mana scalings so mana is essentially AP for him and then tune him so that he's a tanky DPS mage that can output a lot of damage over a long fight but has no real burst.
Or maybe "rip off" Dota and give him Rubick's ult so that he can be the master magician that can instantly replicate a spell if he sees someone else use it.
Maybe give Ryze four low level spells that work like the magic system in Eternal Darkness with him chaining his spells in a certain order to make an additional but powerful effect?