I was discussing this with `Ishq a few days ago. I think it's time that we start devoting some serious thought to this. Thus, a whole thread regarding the topic. This may very well be our first major gameplay change. Think carefully.
Basically, a flying alien would provide the alien team with a method of taking down jetpacking humans, and provide some aerial harassment. You can think of it sort of as a marauder, except it works best in large, open spaces, and is better at picking off lone humans than structures. The alien would probably be slaughtered easily in enclosed areas. Like any other alien, it will have to swerve around to avoid most weapons. Projectile weapons like the pulse rifle would probably be useless against it. Your best bet would be to use a flamethrower when it comes close, or a chaingun from a distance.
Let's give this thing a name. I propose that we call it the vulture. If that doesn't sound alien enough, remember that the word granger means "farmer" and that the word tyrant isn't even uncommon. I'm going to go with "vulture" for now, unless someone else figures out an even better name.
The method of flight should not involve insectoid hovering. It should have fairly large, membranous wings that it flaps to produce lift. This would distinguish the vulture from the human method of controlled flight. It would also allow the vulture to move really fast in the air, at the cost of being hard to control. If you stop moving at any point, you should start falling towards the ground quite rapidly.
Flight controls:
Forward: move forwards in the direction you're facing! You don't fall downwards as long as you keep moving.
Left/right: turn! No, you cannot strafe. How does that make sense while flying?
Backwards: no. How the hell do you fly backwards?
Space: lift. Pressing this will move you upwards. Thus, if you're not moving forwards, you can just keep hitting space to make you go up a little. It's not like hovering though, because you have to keep hitting it.
As for game mechanics:
Health: 100 for regular, 125 for advanced.
Cost: 3 for regular, 4 for advanced.
Stage: probably stage 2.
Size: body is somewhere between the basilisk and marauder. Wingspan can be much wider.
Speed: slow as a granger on the ground, but the fastest alien in the air. Do not allow it to fly in vents. Give it a different bbox while flying. Perhaps allow it to wall-walk in ground mode.
Attacks: melee attack is probably auto-hit, since it's going to be a bitch to aim while flying. Give it decent damage, but have it take a while between attacks, sort of like a less damaging goon chomp. The advanced form can gain a recharging projectile. Perhaps the projectile has a support role? Give it a wide-area spit blob that slows down humans?