Some great ideas here - the argument "A5 or A8"? would probably go to the A5, I suspect, for 2 reasons. First, it would appeal to GCR area afficionados as well, (although they only appeared in the NER area after Grouping, of course), but secondly, getting the bogie arrangement to work whilst still getting the model to 'look right' would take some serious model engineering skills. This argument also works against the B16, H1, NER Atlantics, A7, the original A2 etc.
With the J27 having the same 8' x 8' 6" wheelbase as the Midland 0-6-0s, I'm surprised that Bachmann haven't yet offered one - the most expensive part, of R & D, the chassis, is surely already pretty much 'to hand'.