The plan at the moment is for HTC to update all the really old AH1-era stuff before starting on the AH2 models. I say at the moment because HTC do tend to change their future plans from time to time, as with the P-38 series. So unless they change their mind again we can expect to see (though not in this order) the A-20G/Boston, Ar-234, Ju-88A-4, Ki-67, Me-163 and Tempest next. Recently Waffle has been updating models at a rate of about one a month, although bombers take longer than fighters. So that is roughly the next six months taken care of, although if HTC decide to change something else in the game that needs artwork then Waffle might have to take a break from new aircraft in order to do this. After that my guess is HTC will tackle the early AH2 stuff including the B-17, B-24, Ki-84, P-38s, P-47s, Spits, 109s and 190s. So if you want to start on a shape that will have a future of more than a year or two I'd probably avoid all the above.
If you really like skinning bare metal planes that doesn't leave a lot. The B-29, Ki-43 and Ki-61 are all that come to mind. However all three of those have active skinners.
Edit: The B-26 is a fairly recent shape that has no one skinning for it at the moment.
