Another plot could be that Quinn and Rachel's babies are switched at the hospital by mistake. They then have to deal with the emotional turmoil of bonding with the wrong baby at first and then the process of getting their real babies back. This would involve a lot of heart - wrenching moments as well as some mystery - solving to figure out what really happened at the hospital.
One plot idea could be that Quinn and Rachel become unexpected friends during their pregnancy journey. They support each other through the various challenges like morning sickness, choosing baby names, and preparing the nursery. Maybe they both fall in love with the idea of their babies growing up together and start planning playdates even before the babies are born.