I would like all of the actors from Fant4stic to become part of the MCU in other roles. Kate was already in IM2, so just bring her back and say she was really Julia Carpenter or someone like that. Then bring back Tim Blake Nelson as the full-fledged Leader, who's been chilling out in the Vault with his giant head since the events of TIH. That leaves Miles, Jamie, Michael, Toby. Those shouldn't be too hard.