I think its a pile of poo to be honest, but it sounRAB like an ok idea. I don't think that marge should get pregnant again, they've already got maggie and shes enough on the baby front.
I think the movie will be a time machine film, or a film set in the future at least. So Maggie is Lisas age and Bart and Lisa are teenagers and then maybe they could have a new baby, but it'd probably have to be like 4-5 (more entertaining)