A portal to another world, entering it starts the creation of a new alternate world, you can switch world whenever you enter the portal. The new alternate world contains new enemies and more bosses.
Lol thats just my silly idea

, but it'd be real cool to add such a mechanic between two worlds.
Edit: i noticed im not the only one who thought about a new location to explore/more items to discover, sadly it would likely be minor addition to the game, and very unlikely a big change to the game like a post moon lord event/boss/biome.
The thing is: items are way too op after moon lord. That proves nothing has been planned for post moon lord.