I don't think it would be OOC for Enzo to want to be human. His character is motivated by love and loyalty. It has nothing to do with his power as a vampire. Bonnie is his everything and he has never had someone who loved him in return the way he loved them. I could see him wanting to be human and live out his life with Bonnie and have a family of his own.
I agree that Bonnie would never want to be a vampire though. I don't think it would be a choice though if they did turn her (which I doubt they would). She could always end up a different supernatural immortal though since she isn't a witch (or at least has no powers now). Maybe she ends up a Siren
.