Future Nappa's whereabouts...
Back into the future, Future Nappa will come back to Bulma. After decimating the Androids and Cell, he will remember Android 16, and how he did become good in the past timeline. After retrieving his pod, he will convince Future Bulma to re-program him, just like her past self did. When Babidi arrives, the battle between Nappa, Android 16 and Dabura will be sufficient to allow the revival of Majin Buu. However, Nappa and 16 will be able to make Buu perform the same Heel–Face Turn that he did in the past. The three will then be the protectors of Earth when Goku Black comes around. The fusion fighting Merged Zamasu will either be Naphan or Merged Nappa.
Because it would be both fitting and awesome.