GOOD (easy changes)
1) Fireball Hotkey might be better to utilize "A" instead of "D", it just feels more natural for WC3 to attack with A, or you could just give an option for both players to change it (just have 2 fireball spells with different hotkeys)
2) This is very minor but when using Highlander it bothered me that he had ice spells and the fireball attack, it would fit a lot better if his basic attack was "Frost Bolt"
3) Volatile Sphere has a longer travel distance than a cast range, IMO all spells should have their Cast Range set to 99999(max) and let the players figure out on their own the distance to cast them
4) Every mage could definitely use a 4th spell (not an ultimate necessarily) to give them more depth in their gameplay.
5) Blood Mage hero would be awesome to see with a unique kit to steal life force from other heroes.
BAD (Hard Fix)
-Looking at your triggers, reveals that all the spells are not MUI or at least MPI, this means you could never increase the player count.
-Players can't pick the same Mage, again due to spells not MUI/MPI.
-2v2 or 3v3 will never be possible unless you fix what I mentioned above.