I don't know how close we are to modding 1.9 yet, but I have an idea in mind:
Something simple; random gusts of wind, and hot air rising from patches of lava to aid in the flight of Elytra by blowing the player up into the air (while flying, not falling or by just building up to and standing in one.)
Also, being able to build vertical fan platforms that create artificial gusts for the player to use.
And, while we're adding infrastructure to the air, why not make hawks, vultures, and other flying mobs to attack the player? How about skeletons being carried by large eagles? Or not. The first bit's pretty alright on it's own. A little added excitement couldn't hurt though.
I feel like adding a third paragraph, but I also don't have any more ideas. Fritters. Fritters fritters fritters. Spam fritters.
Well, whether or not it's OP depends on how it's implemented. I was thinking that the "gusts of wind" would be like randomly generated entities that exist temporarily then disappear, and only at a certain height. This means you'd have to still build and climb a tower to get up there. You'd also have to glide into the gusts of wind, so you'd need to at least put some effort into it, and you could just be very unlucky if you don't catch any good air.
The idea of Thermal Vents is more... that these "gusts of wind" are more likely to appear above lakes of lava, and can appear lower down. Hot air rises, of course, so this idea makes some sense.
Being able to artificially create boosts with fans should be reasonably difficult, really. Probably more of a mod that uses RF and a multiblock structure.
The idea of the flying enemies (such as those birds of prey) are indeed for the purpose of making flight remain challenging, yes.
I probably should have been clearer in my original post. :|