I've heard a lot of theories about what happened these characters, so I've chose my favorite or The one that makes the most since.

Barry - I think Palmer retired as a Frontier Brain and Barry took up the position, He must be a powerful trainer if he is the Rival of Lucas/Dawn!

Silver - I really think Silver would want to do something good instead of Evil, I think he's begun to travel the world, It would be really cool if they gave him a cameo in the new games.

N - He's dead. (I'm not crying, you are ;w;) It had to of happened, It explains his absence in Masters, and even if he comes, He probably gets killed after. And i think we all know who kills him.

Hau - I don't think this has happened yet, but it will. Hau will most likely become the Kahuna after Hala retires (or dies ;w;), I also think they should make him dating Lillie canon.