This is just a super quick suggestion so dont hate if its short!
Ok so the other day i thought, what biomes need to be added that everyone overlooked?
At that moment i was reading the Hobbit and it said that: "The Last Desert is a mysterious, possibly fictional land in Middle-earth.
Hobbit folklore described the Last Desert as being a realm at the very eastern end of Middle-earth, inhabited by wild Were-worms.
No factual accounts of such a realm exist. However, Hobbit folklore frequently had some elements of truth in it, such as the stories of the Oliphaunts. It is possible that in the unexplored lands east of the Orocarni or in the Far Harad, there was a desert realm inhabited by dragon-like monsters."
Now on the map, The Last Desert would be located Far FAR FAR east as it is the most easternly known thing that we currently know of, i think a good place to place it would be at the south-eastern corner of the map beyond Rhun and below Mordor.
"Tell me what you want done, and I will try it, if I have to walk here to the East of East and fight the wild Were-worms in the Last Desert," (An Unexpected Party, The Hobbit)
It Would basically be a much drier version of the desert of Harad as in real life it is located at the Gobi desert. it would be inhabited by nomads of the Last Desert and Were-Worms, Nothing else as the Last Desert isnt meant to be hospitable.
The Nomads would have a clothing design simular to that of the Harad Trader Robes and their only structure would be a Last Desert campsite, with houses around a fire made simular to this:
And for alignment im completly open to advancements and suggestions, currently im thinking Neutral with all factions with the exeption of Rhun and Rhûn was, especially during the Second Age, a land where trade was a main part of their culture--especially around the tropical regions around the north of the Sea of Rhûn, where wine, seafood, salt, and tobacco were produced and traded throughout Middle-earth. so im assuming that they have good relations with the Nomads of the Last Desert. As for enemies i assume another far eastern faction as they would not meet very many other people other than the Easternlings.
Anyway i hope that you liked my suggestion!