Yikes, you're gonna get some friction on this.

Well first of all Yagluth isn't the end of progression, there will be more after him. Presumably after defeating Yagluth, in future versions, we will get Mistlands events.

But allowing all events after the final boss (whatever/wherever that ends up being) seems rough if there's no player consent. What I would suggest is each tier of material being used to build a sort of beacon that allows the event from the previous biome. For example silver can create a beacon that will allow the swamp event to hit after you've killed Bonemass.