That's not gonna happen. Side girls gonna stay side girls in Eternum.
Well i don't know about future of this but in old one we only had things with main girls or all girls just become main,as far as I know there was just 4 main girls till half of the game and other 3 just become main somehow,and specifically for these 4 we had rival(weak )but antagonist was there to steal them away,i doubt someone had let them do that ,and for others it was just our choice,so i think side girls will join harem in the end but not sure