Mauville is the current endpoint
The current build includes every route and interior from Littleroot to Mauville and three gyms. Your practical completion marker is earning the Stone Badge, Knuckle Badge and Dynamo Badge.
What to do after the third gym
Once all three badges are in your case, visit the second floor of a Pokémon Center in any town. Speak to the woman at the counter to rematch trainers. The feature lets you keep battling, level teams and pursue a shiny dex while waiting for the next content chunk.
You can also use P2P trading, breed Pokémon, grow berries on their eight-hour real-time cycle and explore for hidden items. The Game Corner exists, although some listed Pokémon are currently out of stock.