Allow skulks to dash by holding a direction and tapping jump in the air. Make the dash about 1/3 the potency of leap, and only allow it once until they land. Incorporate airspeed acceleration into the dash.
Make ceiling climbing temporary. Prevent skulks from hanging on ceilings without resting on walls or floors every 15 seconds. Make one second of rest fully recharge their ability to climb for another 15 seconds.
Let skulks charge and grapple marines. Don't confuse this with knocking a marine over (which is boring and tedious). A skulk that ambushes a marine should have a secondary bite that attacks slowly, and is far more predictable than a regular bite, but that if successful will only allow the marine to knife the skulk to get away, by killing it.
If the gorge is not the alien commander, then give him a new job. Let him be the bait, or the battle priest, or something. Stop making us wait to drop chambers - it's boring, and one of the main reasons NS didn't last even longer than it already has competitively.
Perhaps let the gorge mutate existing structures to do other more useful tasks, for instance a DC that was mutated could act like a barricade having a great deal of health, or a mutated OC could perhaps be manned by the gorge to defend a location. Make the gorge's life dynamic again.
The fade was never intended to be an acrobat - that is the role of the lerk. While I enjoy blinking around like crazy, I see the NS2 fade as more of a teleporting assassin than a super-lerk. Give the fade back his teleporting ability, and make his eyes glow red. Fades are the main assault force of the alien "tribe" - this is a survival/horror game - make them more atmospheric.
Lerks should be the acrobats of the game, and should excel at everything movement-based. They should be the creature with the highest skill cap.
Give lerks a sort of aerial dash maneuver. Give them true freedom in the air - you could give them somersaults, barrel rolls, whatever you feel like. A good lerk should be able to outmaneuver almost any situation, but should be a glass cannon as they currently are.
I would keep gas on the lerk. It's great for breaking up campers, which makes the game more fun.
Stop the onos from climbing ladders. Make a lot of areas in maps he can't get through. Then make him truly dominant in the area he does decide to gestate in.
Let the gorge transport the onos via that canal idea from several podcasts ago.
Let him tank jetpack fire. Require a serious, perhaps even specialized weapon to kill him, and a fair amount of teamwork. He is the maximum tech of the alien force - he should take more than one JP/HMG to kill.
Again, give him atmosphere - he deserves it.
Marines
In my opinion, marines should keep rail hop, but if you won't do that, you should at least give them alternatives - and good ones.
The major point here is that a marine should have the ability to get out of an ambush if he plays well. If that's via airspeed, so be it, if you want to include a minigame of wack-a-skulk with the butt of a gun, then that's fine too. Whatever it is, it needs to give the marine the option to get out of a tight spot.
You could give bullets proximity knockback so that skulks within 3 feet were kited by good accuracy. This would force ambushing and stop stupid players from straightlining into marines with guns, and it would create a sense of banter, which is vital.
You could let marines roll out of harm's way, say a skulk was dashing toward a marine (described above) and the marine saw it, he could press d (right strafe) and tap duck to roll out of the dash - the roll would have to allow him to get up immediately though, as the skulk would be on top of him.
Elements like these will be a start toward making the game competitively viable, in addition to just being really pretty.
