Obviously RuneScape itself could never be open-source — it relies on users performing repetitive actions and interacting with NPCs in a way that means open-sourcing it would enable cheating. So an alternative would have to be engaging in a human way that software can’t emulate somehow. In a non-PVP area, you could just ignore human players completely.
Truly open-source would mean that any surprises in quests and whatnot would be public knowledge before they’re even added to the game.
Being truly decentralised would mean that moderation would be effectively impossible. Updates/rollbacks/etc would require near-consensus. The freemium model would no longer be applicable. Real world trading of any tradable item would be inevitable, and playing the game could be someone’s full-time job.
There are so any issues with making a game like this work. But also, it’s worth considering what could be possible that would previously be unthinkable.
With an open-source game, the default app could be replaced with a customised app to give a different user experience of the virtual world. Maybe even a text-based one.
Decentralisation would mean that, assuming the project is hosted on Safe, law enforcers would be unable to influence the game-play. It could be made closer to real life than would ordinarily be acceptable. For example, a character’s death could mean that they restart at level 1, which would make the game more thrilling to play.
Instead of having multiple servers, the game would effectively be run on 1 massive server. So instead of the same world being duplicated hundreds of times, the map could instead be really really big.
Maybe when I get good at programming games I’ll try to make this monstrosity. Any other related ponderings welcome.