I think the ending is perfectly usable for another story... or even... it could be Goro's travels while Zero two and Hiro are gone well.. You know... It still has potencial for expansion. If well written.. it could end up pretty well.

A special of Goro's travels would be good.

Also a possibility. Maybe a complete story branch (not just cutting at ep15 it that's what you meant)

I really enjoyed it up to episode 15 or even 20 or so but it got a little out of hand by the ending. I'm happy where it left off of course, yet an OVA would be great