I'm not sure if it would count as "sandbox," but I'd love to finally see an official Pokemon MMO release. Something with every region, and travel between them - each with its own Pokemon League, starter Pokemon, gym leaders, and wild pokemon available.
On character creation, you would name your Trainer, customize their appearance from selected options for gender, face, hair, outfit pieces and the colors of everything. You should be able to create any trainer archetype from any of the previous games with the options available, from Lass to Bug Catcher to Medium to Ranger, or something entirely your own.
MMO functions like friends lists and chat could be reflected in-world by your character's C-Gear, and you should be able to form parties of two or three to participate in Double and Triple Battles. In fact, being in a party would change all random encounters in tall grass to match your party size.
Larger groups could be required for high-level content, and would probably involve splitting up to accomplish separate but linked objectives to do things like shut down Team Rocket HQ, or explore an expansive, procedurally-generated ruins complex filled with inanimate, mystic-flavoured Pokemon like Unown or Sigilyph or Chandelure. Of course, anything wild should be catchable, except legendary Pokemon - these should be treated as boss encounters, such as three trainers teaming up to fight Mewtwo, who is three times their level, and making him faint so he can safely be put back in his capsule at the lab before he destroys Cinnabar Island and sinks it into the ocean.
You should be able to trade and battle with any trainer you meet in the world, and use the PC in your character's room to trade and battle with friends who started in different regions, even if neither of you has the means to travel to the other.
Beloved characters from the anime should populate the world alongside normal NPCs, with each city, in Kanto at least, having its own Officer Jenny and Nurse Joy. Team Rocket should actually have Jessie & James, and Butch & Cassidy, etc. Ash should be the current champion in the Kanto League at release day, and Gary should still be an asshole.
The world should be rendered in full 3D and traversed in real-time like most other MMOs, and a player who encounters a wild Pokemon should enter a turn-based battle scene visible to other players nearby - their character controls are locked, their first Pokemon is sent out, and the battle proceeds turn by turn, just like in the single-player Pokemon games. Anyone nearby would be able to watch the action, and this would extend to trainer battles. Events like the Pokemon League or Nintendo-sponsored tournament events could take place in an arena with bleachers where players can log in, buy a ticket from an in-game counter (with game currency, not microtransactions!), and sit with their favourite Pokemon and watch the tournament like a sports event. Doing so could even raise the Pokemon's happiness. Pokemon shows/talent competitions should be treated similarly but in a more fitting venue like a theatre.
Classic game systems such as gyms, breeding, egg moves, TMs&HMs, etc should all be present, though Pokemon should be allowed to forget HM moves when learning something new. No-one likes retiring a Pokemon to the box because you needed to cut a shrub down and cut is a bad move.
The Pokemon League tournament should be held at regular intervals, where anyone with all the badges in a region can enter. The winner would be granted the right to challenge an NPC of the current Champion, and if the challenger is victorious, their character becomes the new Champion - their character model, name and current team are created as an NPC and fighting that NPC becomes the prize for next season's Challenger. Nothing happens to the new Champion's progress, no Pokemon are gained or lost, though prize money and items for the winner are totally a thing. Throughout their term as Champion, the player can visit Victory Road to update the levels and move sets of their championship team, should they level up or learn new moves after winning the championship. The current Champion should have a special title display on their name, and all previous champions should have one too. There could even be a Hall of Fame on Victory Road where players can go to see a list of all the champions in chronological order since release day, select one from the list and view their championship team they used to win, as well as the potentially updated championship team they used to defend their title. Each region would have its own League and its own Champion, and after each season, the Champions of each region would battle in a round-robin style tournament to determine the World Champion. Champions who are unable to be present for their matches would be replaced seamlessly with their NPC counterparts, in order to allow the tournament to go on.
If a full-featured MMORPG like this were released, I would pay $15/month to play it.
Wouldn't you, Escapists?