I think it would be quite fun if heroes had few tiers of abilities - one would charge at very fast mana, other at average and yet another at very slow.
On offense you get to choose which one you want to fire at which point. Fire now with weak effect, or wait couple turns hoping there’s going to be enough tiles to let you charge for stronger effect.
On defense you just set your heroes to fire at certain tiers all the time (given how little control over defense you already have).
Still the above would probably require toning down overall heroes power a bit as we now suffer from inability to cast any specials (because defenses are too powerful), rather than from lack of good specials to let us counter things.
I also described few ideas in my Restore Fun in Raiding - ideas and discussion