If an MCU film that introduces Rachel follows Days of Future Present, her character arc would be far more than just being Franklin's love interest. That's such a reductive way to view that story. It would make her and Franklin people marooned in the past, but while Franklin is born and anchored by his past self, Rachel isn't and has been replaced, a mother who rejects her and because of that she can't be with the father who wants her, it would show just how much the Hounds changed her, it would show her deciding to go and forge her own path with Franklin's sacrifice. Rachel's arc in Days of Future Present is a beautiful one that touches on so many elements of her character, including the scene where she, Franklin and baby Cable try to make a family so they can have a moment of normalcy they would've had if not for Ahab and the Sentinels. It is the tragic love story of two damaged people and their place in a world where they don't belong. Honestly, I personally feel it is one of the most deep yet underappreciated X-Men comic arcs.
Hell, that very last panel is probably my favourite Rachel/Franklin panel. She is Phoenix.