The Spectre was a pretty awesome vehicle, but if I have to be honest, I understand why it didn't show up in Halo 3. The infinite boost coupled with the Plasma Cannon turret did make it overpowered. With the new gameplay mechanics, though, I think it would be fun if it made a comeback.
Promethean Vehicles I think sound awesome. Jump in, and it modifies its structure around your Spartan. A tank firing heavy Incineration Cannon shots, maybe with a Pulse Turret or something, would probably be my new favourite vehicle. The only trick is to balance them with other vehicles.
Dual-Wielding I kind of understand, although I have a slightly better idea. Have a variant of some loadout weapons (Plasma Pistol, Magnum, SMG, etc) where it's dual wield. More firepower, but longer to reload, less aiming, and no grenade-throwing capabilities. It'd probably be awesome, and dual Needlers would be the ultimate Campaign/Ops weapon.