Personally I hope they will be random. The trouble with linear campaigns it you would really have to concentrate on a specific theatre for continuity. I would hope the ETO, PTO etc will get equal exposure. (Or have I misunderstood what you meant)? A linear campagn set up sounds to much like WW2OL to me and adding aircraft at the historical times when perhaps the virtual campaign is going in a totally different direction to it's real life counterpart has always seemed wrong to me.
TTFN
snafu