I think the best way to balance out the problem regarding mage wings (abusability by other classes) is that they get a direct buff from the mage class when in use, which specifically allows them to be used.
I had an idea regarding this: the beginning of flight time for these wings costs mana, but the timer can be extended by dealing exclusively magic damage. Perhaps some kind of (damage dealt)/(modifier) = (seconds added to flight time) equation.
I'm not sure how feasible this is regarding theme compared to the current state of the game, but it's theoretically quite simple to implement.
This way, other classes would get the initial flight time of the wings balanced to their tier of progression (albeit still paying the initial mana cost), and only mage class would be able to utilise it to its full potential.
I imagine there would be an upper limit to how long you can extend it, though this might not be necessary due to the fact that you will eventually kill all enemies on screen. Statue enemies should not contribute to the timer to avoid cheesing.
Thoughts?
(Anything after this point is an edit to my original post.)
I also like the idea of giving mages more of a glass cannon playstyle, with high manoeuvrability. Different spellbooks for movement along with mana-based movement accessories could easily make the mage class feel more unique compared to Ranger (imo), and I feel that could be a change for the better. The leap and magic wing books don't sound too bad, they're a good start; perhaps a variety of teleportation-based spellbooks could also be implemented, which can have varying distances? Could be an alternative to the RoD, instead costing mana and resulting in mana sickness on use.
Just some ideas, is all.