What you are describing is pretty much exactly what Realms was. it had a core “campaign in a box” structure (the spider tower story line), but you could totally ignore this and just go faff off and grind goblins and kobalds and explore what I think might have been procedural generated dungeons. Idk. I was very young. But I literally put dollars in an envelope and mailed them off, and weeks later, got scenario unlock keys back.

It would be really cool if you could extend the core world with community contributions or story lines, like extension campaigns or open world scenarios.