Now THAT would be great! Vintage head, 200X head and long neck. Hopefully they can do it...
Also, having said that, there was also more drastic differences between the 80s Zodac and 200X Zodak than there are between the Rattlor variations. The biggest of which being his skin color and the tattoos of the 200X version, which are things that cannot really be easily facilitated through optional accessories.
Rattlors Design isn't really THAT much different from his old design. The biggest differences are the armor and the face design. And that issue can be easily solved by giving him removable armor and an alternate head. They will likely handle his neck via the removable head option, so just make a neck extension piece, and give him the 2 different heads, each of which can work with or without that extension piece.
I think the build really isn't an issue since MOTUC isn't making really big characters like Ram Man, Clawful, Whiplash, Beast Man, etc. Unless they made his upper torso a little wider, with bigger arms like the 200X figure. But this is unlikely.
Sure, his build changed some between the 2 versions, but they can probably just find a happy medium between the two (plus they'll probably want to reuse parts anyway, so the build will likely be more "neutral".... even if they were to make the 2 different versions as 2 separate figures, I wouldn't count on a whole new body sculpt for the 200X version anyway).