The Ancient City Portal (2 suggestions)

So we have the warden boss, and as we know a warden is one who guards or keeps.

So If we kill the warden (or a couple depending on balancing), we get a way to light the portal, leading to the Prison. The Prison is a new dimension that is basically the Deep Dark, but much MUCH scarier and more dangerous. In The Prison you can fight The Prisoner, a big boss about the same difficulty as the bedrock wither or higher. Once Defeated, the prisoner drops a special item, this item can be used to craft something, or make a tool or armor piece NEVER lose durability.

On, the contrary, the portal could lead to a utopia of sorts, a dimension quite opposite to the deep dark. With new building blocks and no hostile mobs, maybe a special structure or two (like a special village or something). This would give us things like being able to bring the floatater back from April fools 2024, and give us calcite building blocks like Calcite bricks, tiles and chiseled Calcite.