Not gonna lie, this eternal fuel source thing is sounding a lot like a strawhat dream. Vegapunk has a role and a dream, and has asked Luffy to take them away from the island on the Sunny. I thought Bonney or Hancock would be the main possibilities, but I'm not far from unironically believing in Vegapunk for nakama in some form. I need to see more interaction between them and Luffy, as well as their backstory. A tragic flashback and a villain for Luffy to fight (CP0 or new WG force?) are all they need to be a very viable candidate.