Spells too instant. No charge time, no drawbacks.
But what if spell needs time to charge up?
What if it depends on mana cost..?

There's could be new stat to improve, something like "cast rate"("Mana per second" or something)

And mana could go out into spell smoothly

Uh, there's also those "instruments" to cast spells, like sword, mirror, wand etc...

Cooldown could be interesting too

Well, here's idea
And
it needs time to think about it, I guess...

P.S.
Evokers turning hands up before casting
What if casting would have animations like this?..