As for gameplay, here are some ideas: myst/Riven style puzzle solving to tell a story. Some big plot. But in order to move the story along, solve puzzles to maybe restore rides to working order, collect clues or pieces of something.
Another idea: instead of just restoring the rides as they are in the park, each ride when returned to functioning order becomes a portal to a larger world of the ride's theme. Jungle Cruise takes people to the Amazon River, the Nile River, etc. The Haunted Mansion is a portal to a larger Bayou area. Space Mountain literally is a portal to different space ports.
As for MMoRPG, Unreal with the Metaverve might be able to handle it, especially with Nanite. However, if they make it a desolate place like Riven, I would not complain.
Honestly, I would be happy with a community effort to simply recreate the whole park in Unreal