@Hael
The thing is that illusion is sort of split between two categories (Telepaths can make you hallucinate, yet Alterists could physically change the appearance of the item and disguise it as something else, so that you aren't hallucinating at all). I'd also say the traditional Illusion-type powers of invisibility and such fit under Alteration, further meaning that Telepathy isn't just the same thing as illusion magic. So illusion as a combination of the two and a category of its own would be overkill, but pretty powerful.
Siigh. Your missing the point of the categories. Magic in this RP is not actually divided up into types at all, rather the types are just an easy way of organizing them. If you are an alteration wizard and you make yourself invisible, the spell is both alteration and illusion
already, therefor to combine it with illusion would do literally nothing.
They are actually quite powerful even in combat, with the ability to enchant the land itself to make the arrows of friendly archers light up with magical flames and incinerate anything they hit, for example. Or just empower everyone nearby.
Any mage of any category can already do that, it is a basic skill, as I keep saying. The categories are only a guide, not rules. Nobody is limited by any of the Magic Schools. It doesn't matter if something fits into a category or not, it can still be done as if it did.
Vec