Where to start, where to start.
Second the 335, although I'd like to see that as a perk plane in the MA.
I'm thinking of all those really whacked out Luftie ideas that never got off the ground, or did so in only limited numbers.
The HO 229 (any of you remember _Secret Weapons of the Luftwaffe_? The 229 was my favorite ride).
All of those great 262 variants (how about the one with the 75mm cannon that they had to abandon because the muzzle flash blinded the pilot? Or the two-man glass nose fighter bomber (I've a model of that one waiting on my shelf)).
I'd also like to see the ME 410, although I have high hopes that will one day make it into the MA, since it was produced in numbers equivalent to the 110G.
I think this idea of a "what if" arena is a fantastic one. I can't see it happening any time soon, since HTC is geared toward the MA planeset (and rightly so too). But it would be something interesting to begin work on.