Maggie Roswell still does occasional voice work for The Simpsons, right? Since Fox probably isn't going to pay Winona Ryder to come back, they could just have Roswell voice Allison.
Ralph? They can still bee friends, right? And it's been years since I've watched, but does Lisa still have the on-and-off friendship with Nelson? Post "Lisa's Date with Density," they've occasionally interacted on a friendship level. There has to be some certain circumstance for Lisa to interact with someone: a girl is as good on the saxophone as her, she gets a pen pal from Brazil, or she's invited to join Mensa. But it's always temporary. Lisa's never really had many friends outside of...well, Marge.