I thought about that. Maybe they can use a wish to either ask for eternal life for Rix or a mortal life for Nuah. However, I personally would prefer for Rix to not become immortal or Nuah to become human. That's a little bit too fluffy for this story in my opinion. I think it'd be more poetic if they ended up together the way they are now, having to accept that their time is limited (and also maybe having a child) but going for it anyway. Also, if Rix had eternal life he would have to watch all of his friends and family members die. That can't be pleasant.
Indeed... but we will see, right!
Can't Rix ask God for eternal life so he can live on with Nuah?