Luke and Rey are the last remaining Jedi and since the Jedi actually becoming extinct would be disastrous, they have to reproduce as a baby with two incredibly force sensitive parents would undoubtedly be a Jedi. Cue Rey being shocked at first and Luke reassuring her that she doesn't have to do this but then she finds herself wanting to- really wanting to. Give me Luke going all gooey whenever he sees Rey cradling her bump and Rey hitting Finn for acting really weird about it (Poe thinks it's great, he's got a soft spot for kids). And Leia being so shocked but ultimately happy that her brother has finally got something like she had.

(Obviously my headcanon for this is that Rey isn't a Skywalker but whatever floats filler's boat...)