The set-up comes directly from the story she told me about:
*Harry has an accident that sends him back in time and de-ages him.
But I would like to see it happen in the time when Lily is still friends with Snape and thinks that James is an arrogant git.
*Toddler!MostlyAmnesiac!Harry (all he remembers is his first name) appears in the Great Hall during a meal.
*Something about the way he arrives leads Dumbledore to recognise that he's from the future.
*Dumbledore and Lily both recognise where Harry's eye colour comes from, but Lily flatly refuses to accept that his hair comes from James, so she settles on Severus as being his future father since he has black hair and they are pretty close.
*Dumbledore notices who Harry's real father must be, and asks Lily to look after Harry while they work on how to send him back. He hopes that having to care for their child will bring Lily and James together. Lily sabotages this by insisting to Severus that he help her take care of 'their' son. She's very insistent.
*...and I have no idea where the story should go from there.
Anyone else think that could end up as something other than badfic?