Good Friday everyone! Lets post some fun guesses as to what you think is going to happen.
***ATTENTION SPOILER ALERT IF YOU HAVE NOT SEEN THE BETA CLOSE YOUR EYES AND READ ANYWAYS BECAUSE ITS AWESOME***
I personally think its a big boss possibly Charlie who will prevent the final seal of the constant to being broken. Once defeated we could see our way into a few different potential situations at least this is all in my opinion anyways.
1 . Possibly given the option to enter Hamlet or something similar. The only problem with this would be it takes a lot to get to this stage and some people might find work arounds thus leading to Klei not wanting to bring previous content from another game over for it to feel "re-used or recycled" however this could still be fun none the less. The reason I am guessing this is because Wagstaff is attempting to "Free" them from the constant as seen in the beta but it could also be a trap for Wagstaff to bring us to a new "Plane" of existence as stated by Wigfrid when she reviews the portal itself.
2. This could Release all of the negative energy in the world and creative sort of a release the shadows situation making the world go into hard mode so to speak. Everything becomes shadows bosses because extra hard with new moves. I highly doubt this would happen but it could be a possibility.
3. The portal opens and it rolls credits or a short animation that shows us a glimpse into a brand new game Klei is working on Perhaps Don't Starve Together. 2. 0 but I highly doubt that but it is possible. The reason for this would be because right now both the overworld and the caves are pretty much filled to the brimm with biomes. If they decided to add a new Shard (World) For example hamlet you might have issues with lag / world size and just in general complications. There also have been numerous talks about how hard it is to code for Klei's older games and most developers usually upgrade to a new game to be more up to date with the times. However I highly doubt this but who really knows it could happen.
4. The door leads to a wormhole or teleporter that opens the constant that allows you to travel to a new place entirely. Some things I could guess would be the Moon, Perhaps back in time as well, Lead to the Forge, Lead to the Gorge, or even leave to the weird unknown "BORGE" also known as the shadow realm.
Overall these are some ideas I have let us hear your ideas 
Perhaps Klei will take a few things from this and make something of it. My honest opinion, I think its a gate to the Moon or Hamlet or a combination of both. Because everything in the Archives tells us its from the Moon or "These people loved the moon" as well as the markings resemble that of Hamlet.
ONE BIG THING I NOTICED* Was the chandelier in the Archives if you look at it and then look at Hamlet they look like one of the only true Items that offer a 3D look into DST other than Tentancles and Rocks in the Caves.
Not to mention the swirl patterns are the same from Hamlet to Don't Starve Together.
I am always open to criticism so feel free to shoot my guesses down after all as a Content Creator you should be able to take good criticism
