First of all, HTC put up that announcement for a reason...to stimulate discussion in these boards. It takes a heck of a lot new coding to do what they're planning. And if there's lots of negative discussion in the BB, then HTC will know there might be a problem with their concept plans. eskimo2.
As far as enemy opposition, I can only speculate that it might be a combination of AI and manned enemies. But, if there is no one else in the arena, then it would be definitly all AI. In essence it would probably be a co-op mission arena. So you and your squadies might be battling it out with AI CAP or whatever.
So, I have mixed feelings about an AI co-op arena. I can fly IL2 Sturmovik with some buddies in co-op mode. I can fly in campaign mode. I can also do the same with Falcon4.
But, the mission arena, if it were populated with enough manned pilots would be a cool idea.