An interesting idea that aims at addressing the problem with fireball and other spells: 

it would be nice if I can decide how long any spell is being cast with bonus in power for each turn of casting

i would do it like that: if you click left button of mouse the spell will go with default (no delay) power, and if i click right i could postpone a spell - this would make a game really interesting: do i want to wait? how long? Of course, with each turn you risk being disrupted. 

in order not to abuse it i would also increase a cost in mana of casting it depending on time you take to cast it