I think, that right now, the standard flying speed is too slow to fly around trying to get to further distances, but too fast to build detailed constructions in the air. Ofc you can slow it down, which is nice for the last point, but for anything else than building small things it isn't useful.
Maybe we could get more velocities, a way faster one, maybe even an insane one and a moderate one (Between the current two).
Also, it would be great to have some easier to reach key settings, like using [shift] in air as well to speed things up.
I think something like the following would be useful:
No shift hold: New moderate speed (slightly slower than current one)
Holding shift: Current standard speed
Holding shift + scrolling up with the mousewheel: 2X speed (scrolling back to undo, releasing shift will automatically reset speed to standard)
Holding shift + scrolling up again: 4X speed
And maybe an extra buton (e.g. [C] since it is used in some games as crouch) to enable building mode (but you don't have to hold it), since you can't hold shift + mousewheel down all the time 
holy smokes !!!
