Further development of AmigaOS 3.X?
AmigaOS 4.1 68K?
Hmm, the first can't be done (unless you ask Cloanto), and the second is unlikely as it would undercut Hyperion's PPC product.
Hyperion could do it if they wanted, though I really doubt they do.
It wouldn't solve anything though, as you would still get loads on non upgraders and grumbled at for not back porting your software, even though makes use of the new and cool things that come along in a growing (even if slower than we;d like) OS.