Pokemon Ultra Sun and Ultra Moon is not a sequel. It will be DIRECT build up for Generation 8 games.

There will be be NO NEW MEGA'S

Some Legendary and Mythical Pokemon will get their own exclusive Z-Moves

No new alolan forms

Lillie is not coming back

Ash will make a cameo appearance since Ash-Pikachu Hat (Pokemon Movie 20) is in the game

Lycanroc will have a 2nd Evolution