Prarie. And don't explore too far into the Ash_lands or Northwhatever areas.

Its the last area of the map that is currently finished.

The more you explore the more area of your map you will "lock" in that bioms current form. The terrain is going to be generated out 500 meters from the edge of the revealed map and won't get any new content from later patches.

If you have already been all over your map you will probably need to create a new world after the the next big content update or use the Upgrade_Worlds mod to reset portions of your world to actually get any of the newer content.

you can get the mod here
https://valheim.thunderstore.io/package/JereKuusela/Upgrade_World/
or here
https://www.nexusmods.com/valheim/mods/1485