Rolling the romance paths in with each other seems inevitable, if for no other reason than ease of development.
That said, Holly/Ian could have divergent paths for a while where the old route, the Ian/Holly romance from the bookfair, could perhaps be allowed to "catch up" if Ian and her were to get some options to explore sexual encounters with other people for example. So I could perhaps see Eva veering a version of her old route off into a sluttier direction, if chosen by the player, which will then intersect with the Slut Holly romance, if the player sticks with it, at some point. So Ian could have a more wholesome romance with Holly or a more slutty one in the same vein as Ian/Alison sharing or Ian/Emma open relationship.