Yeeaahh there's already effectively 2 or more champions like this. Listed by other people down below. If you give them a low costing damaging ability with quick cast time then it doesn't matter if they can attack. Karthus uses Q unless he runs out of mana, which is essentially this concept just without an option after you run out of mana.
And they don't really want game-breaking mechanics. Game-changing is the goal. Which honestly there are a ton of potential awesome champion concepts in the boards... Ones I'd enjoy far more than recent champs.