God, where do I begin...
Anti Air Wraiths were looked into but not added because the unit Gorgon was going to be used instead for the AA role. Being that the Covenant had way too many ground based vehicles they decided to make the Vampire instead (so both UNSC and Covenant would have 2 air units). The Gorgon, simply put, was a chicken legged mech looking thing with Needlers. Anti Air Wraiths woulda worked, yeah, but too similar to the normal crap Wraith.
Specter, I think woulda been a great addtion, perhaps the final upgrade to the Ghost or something, sorta "super-unit"ish. But nah.
Same holds true for a grunt squad, I mean, Deacon? Why not make them killer with the upgrade being Fuel Rod Grunts?
Or instead of flamers Marine Snipers? Or is that too similar to Jackals?
But yeah, some sound ideas from you guys...
My name is Legion, for we are many...
And out of the smoke,
Locusts came down upon the earth and were given power like that of scorpions of the earth...