ok, well that mixture of air/ground was something that appealed to me, but let's think of some other things that could happen in the air.
tank busting - ok, you already covered that one, with maybe both static targets & real moving tanks
transport work - not in a Cobra, but anything else could be diverted to pick up a squad in trouble or a general who needs a ride, or a medevac priority, or shift some more mortar rounds from base to resupply a forward company, or whatever. I guess that's no fun for the gunner though.
recon - go take a look, and maybe right-click on some installation (like the mission where you find the base in the 1985 campaign, or where you call out the bad guys for the resistance). Why would you just look and not take it out? don't know - maybe it's too big, like you're locating a large ship off the coast, or a supply dump.
here's an odd idea - what if the enemy suddenly switched on their SAM radar, and you have to fly the rest of the mission below 20m - that would give the gunner a serious sense of speed.
Fire support - like cover for a friendly squad being picked up by a blackhawk, while your pilot circles, and you have to pick off enough of the bad guys to make them keep their distance
anything usable in there?