I agree: Magic Missile would be nice.
For the passive, I'd say something that would make this a D&D mage: manaless, but gets a limited number of spell casts per recall per spell (depending on spell or champion level).
It would be very cool if R could be Time Stop, but I'm afraid that would disrupt the "twitch-reflex-pacing" rito is trying to cater to, so that won't happen.
Maybe a Chain Contingency would be better?
Wait a minute - you don't have to decide which spells he can cast!
Just give players a list of available spells, a menu to change them at the fountain, a level limit like the one you wrote, a passive that explains this, and that's it: you have a D&D mage!
This would result in an extremely versatile champion with abysmal sustain (the limited spell casts would act as a very small not-improvable and not-recharging mana pool) to keep it balanced.