Has there been an MMORPG type attempt at some kind of open universe where you could host a server on your own if you wish and it would merely be added to the collective of possible places to travel within the MMO?

Two types come to mind, a DnD Neverwinter Nights type place or something like EVE online. Where there is a "universe" and each hosted space is a planet or solar system or galaxy and players can travel between them using the same characters/ships/portal system and each new server is than just a new adventure or place to go.

I would also assume there were dedicated/replicated servers that housed the characters/inventory themselves so that the environment was decentralized and always expandable.

Not sure thats clear but has there been any such attempts or WIP?

thanks