Input on vehicles: Unless Halo will have more than 16 players, i think air transports should not be featured in multiplayer (for customs and Forge, sure, but not MM). Ground transport is riskier and way cooler, and any map where air transport is useful is too big a map. Never liked the T-Hornet in Avalanche.
As vehicles go, i'd stick to Halo CE-like Shade turret, possibly a human counterpart, the Ghost, the Banshee (balanced, Halo 3 is good starting point i think), the Warthog (MG, maybe Gauss, Halo-PC like rocket variants?), the Mongoose (a tad weaker and slower?), the Wraith and the Chopper (more of a vehicle killer whereas the Ghost is more versatile?).
I'd leave out the Scorpion entirely, it cannot be balanced for multiplayer and it is rather boring in campaign: slow, powerful and tough vehicle. Ain't exactly what Halo requires or should be.
I don't think UNSC really needs an aircraft. Unless you bring the Sparrowhawk from Halo Wars, with alternate weaponry of course. That thing is simply cool, unlike the Hornet (boring, slow, OP) or the Falcon (cool, non-sensical (no armored doors?), good controls(!), ultimately rather boring and somehow doesn't feel very Halo)
