There are some modpacks that have already implemented monsters and bosses for the 3 last tiers, mistlands, deep north, and ashlands. Epic Valheim is one. I think Valheim Story or some such is another.
They also add armor and weapons to be made with those new ores.

Now the land you are referring to is a bug of sorts. I saw a video on youtube about it when you fall off the world and wind up in a desert-like infinite land with not much there. That might be a testing area for devs while they were working on the actual ashlands since it is nicely textured. Who knows.