My suggestion is to allow momentum to be maintained when exiting fly mode.
Currently, when you exit fly mode via pressing the fly key (in my case, F), then you lose all forward and/or vertical momentum and simply begin falling downwards.
If you were to allow this to be maintained, then players would have the ability to switch between flying quickly and cycling to flying slowly without sacrificing several seconds of travel time.
Additionally, this would allow for some interesting "parkour" style challenges on worlds, where a player must launch themselves using the fly feature into a no fly zone, such that they land on a platform.
And on top of all that, it would make for some sweet cinematic shots.
Now, the disadvantages of this feature are as follows;
1. Players would have to "fly proof" their worlds to prevent from players who are in fly zones to launching to areas they are not supposed to be, in no fly zones.
2. The way that players accelerate under fly will have to be changed. Rather than just maintaining a set speed, utilizing "slow fly" will instead have to be set to either decelerate or accelerate to that specified speed depending on the current speed of the player. And fast fly will have to be set up the same way.