I think the first game should feature a theater/theme popular enough that it doesn't have to be marketed to grogs only, but at the same time it shouldn't be a game that I will want to spend most of my future CM time playing. The first release should be a sort of demo of what CMx2 is capable of. If the capabilities of the new engine are going to be more fully exploited with each release I would rather BFC hold off on the really poplular themes until they are ready to exploit the engine to its fullest.
To clarify my point, CM:BO is arguably the most popular theater of the CMx1 titles, but I'm sure most of us wish BFC would have gone back to update CM:BO to take advantage of the new developments in CM:BB and CM:AK. I don't want the first CMx2 release to be the game that I will want to play the most if the engine is not yet ready for prime time.