So now that (with some difficulty) you can fly and cast, I think I'd be a cool feature if some spells knocked players off their brooms. Mostly direct damage PVPs one, because while bombarda would probably knock someone off their broom, silencio probably wouldn't.
Maybe even the skill of the flyer and the caster determines the chance of falling/remaining on the broom?
It's not something I think NEEDS to happen, but I could see something like mid-air duels being made more interesting by this, and it adds a little bit of 'realism' imo.