I think Jet is interwoven into the main story and only gets developed as the story progresses. Sharon is most likely done as you can fuck her anytime you go to her house in the morning with a bottle of wine. Maria is again main story, but I also do not think she will develop any kind of relationship beyond what we have now. Perhaps at the end there will be a sandbox mode and we can just go around fucking the girls, but I think what we have is what we get unless the story unlocks more.
Amy would be nice, but if Laura is any indication, she will always be a side story and never really the main focus.
Seeing as how he has released .9, It should be that he would be completing the game in the last version, Though I doubt this will be the case unless he has been slowly working on those parts for months now. IMO he should try to finish the main story to release a 1.0 as his dev cycle is pointing, then add girls after like some continuing support while he works on his next game, allowing people to keep interest in his work.