You can already assign your mouse buttons to other actions than 1, 2 and 3.
You can also already cycle through (a) inventory rows and (b) which spell is in which slot in its row.
-- All that good stuff is in the controls configuration.
That said, I agree that it would be useful to:
1. Be able to set mouse buttons to spells which weren't in your quickbar AT ALL; and
2. Be able to cycle through 'spell sets' without disturbing the rest of your inventory.
I'm thinking, a way to assign spells in the way dungeon crawlers assign weapons and armour, and then button/keyboard bindings for "Primary attack", "Secondary attack", "Tertiary attack", etc. plus "Primary block", etc.
You could then outfit yourself with up to x number of offensive spells and y defensive, then bind them to the mouse to your heart's content - and leave the inventory quickbar for holding on to anything you don't have assigned in that capacity, like light spells, platforms, traps and equipment.