A round is different than a turn. Currently the UI does not make this clear to the user. A round is a set amount of time. A unit can have many turns in one round. Let's take advantage of this.
I am starting to agree about need a new mechanic for spells. An arcane stat could be used to determine how long it takes to cast a spell. This would be a simple addition. 0 point of arcane means that all spells take a default of 2 rounds to cast. 1 points halves the casting time, rounded to your disadvantage. 2 points cuts casting times by one third and so on. The first point would be in the common pool. The second would be part of the Path of the Mage and the rest would be unlocked every 3 levels from level 4, where you get Path of the Mage.
Spells like Fireball would start at 3 rounds to cast. Better spells would follow suit.
Level 1 heroes need 2 rounds to cast any normal spell. This would prevent noob mages from stopping Fireball at a whim.
Level 2-3 heroes need only 1 round to cast any normal spell.
Level 4-7 heroes can cast any normal spell instantly.
Level 7-10 heroes can cast Fireball instantly.
In this system Initiative is useless because no matter how many turn a mage gets in a round, he would have to wait until the next round to cast the spell. Mage are greatly rewarded for specializing in magic and mundane heroes will not be able to compete. It allows for cross-class units to be able to use magic, but not overpower devoted mages. It would also open up the possibility for mid level and high level units to cast a fire spell that allowed them to shoot a series of 4 fire darts on the next 4 turns.