I want to make it clear that my message is not a list of proposals per se, but a simple list of my ideas that came to mind. These ideas came to me while looking at Feivelyn's beautiful artworks, so they are not really original. Chances are that they are all throwaways, devoid of any quality, but I still wanted to share them somewhere. Hopefully I won't disturb the forum too much.
Last precision: sorry if my English is approximate, I don't speak the language very well.

Let's imagine the Telvannis disctrict as a place separated into two large territories: 

- A lush, fertile territory, used for agriculture, herding and hunting. It's a good place to live, but some creatures can still be dangerous (like everywhere in Morrowind actually). This territory would be composed of two regions: a plain filled with Corkbulb, and a very thick forest composed of many mushrooms and creatures of the night.

- A hostile, dry, almost lifeless territory, where venturing there would mean dying. A place where the Telvannis can experiment, and where the craziest of them can even take refuge. This unique region would be a mass of white ashes and burnt/dried corals. The only living species in this place would be parasitic plants and insects hiding under the ash.

The whole would be separated by a big mountain range, looking like sharp ebony blades: the Boethiah's Spines.
 

One could imagine that originally, even before the arrival of the first Chimers, the area that is now the Telvannis district was rather similar to what it looks like now: a kind of mix between the Azura Coast and the Grazelands. The only difference was for the eastern part, which looked like what I imagine the Telvannis Isles to look like: a rather colorful, stony but coral-covered place. It's not too strange to think that at one time the Telvannis Isles and the mainland shared the same biome. But one of the eruptions of the Red Mountain scattered ashes on a part of the continent. This happens frequently, and the ash deposited on the mainland washes away very quickly with wind, rain, etc. (except in the area of Armun, the Grey Meadows, and Vvardenfell).

However, this eruption was so violent that it caused the creation of a mountain range on the former Telvannis territory (and many debris thrown by the volcano crashed on this area, contributing to the formation of this wall of stone and ebony).

As a result, a lot of ash covered the territories of Morrowind, but the ash quickly left. With the exception of the eastern part of the Telvannis district. The Boethiah's Spines prevented the wind coming from the ocean from sweeping the ash away, and it stayed there forever. The burnt corals were fossilized, and no more flora grows in this area, eternally covered with ash. This one renews itself little, the Spines blocking the arrival of ash from the volcano's eruptions. The ash has whitened with time, giving this unique aspect to this copy of Ashlands. (The whitening of the ash over time is not very realistic, but one could imagine a whole lot of reasons in a magical universe.)
 

A little bit of ash however managed to pass through and settle on the Telvannis district, just above the Helnim Fields. The spores from the Bugherded Hills mushrooms traveled to this area, and the ash served as a rather effective fertilizer for a multitude of mushrooms to grow, forming the Black Forest.
 

1 - Telvannis Isles

The Telvannis Isles, which will be covered with colorful corals and sinuous sadriths, as well as lots of rocks. A place similar to the Azura’s Coast and Sheogorad, but with a unique colorful side. The area should not change from the vision of Tamriel Rebuilt (except that one more island has been added).

Feivelyn Art - An inspiration for the Telvannis Isles.

2 - Boethiah’s Spines

It’s big wall that blocked the ashes of the Dustlands west of Morrowind, eternally pushed by the wind coming from the ocean. I was very inspired by the Atacama Desert (in Chile) for the Dustlands.

It’s also a rather dangerous area. The terrain is very steep and unstable, the walls are sharp, and there has been some light volcanic activity (due to the flow of Red Mountain and the earthquake).

Feivelyn Art - An inspiration for the Boethiah’s Spines.

3 - Dustlands

Inspired by the Ashlands, but with a more desolate and morbid aspect. The Ashlands are like populated deserts, the Dustlands look like a burnt area where everything seems to die. The ash would be whiter, and there would be dead corals, as if the region could have been similar to the Telvannis Islands at one time. The wind in this region would be rather strong (like the wind of a coastal region), and we would see mushrooms moving around, clinging to small stones (fossilized corals) by their roots. Because of the wind that constantly blows the ash against the walls of the Spines, they would be much more concentrated on this side, forming a slope to the coast. There would be constant clouds of dust, so much so that the sky would not even be visible.
One could even imagine that the famous "hive" of the vanilla concept art "Ashlands" could be found in this region.

Feivelyn Art - An inspiration for the Dustlands.

4 - Bugherder Hills

Bugherder Hills, or Corkbulb Plains, would be a large, very fertile area similar to the Grazelands, but with more orange, more fertile vegetation, and with more mushrooms - and most importantly, the famous Corkbulbs. There would be a lot of insects in these regions, most of which are bred to be used as food or chitin.
Except for the area in which I placed this region, it should not change much from the Tamriel Rebuilt vision.

Feivelyn Art - An inspiration for the Bugherder Hills.

5 - Black Forest (Baan Mora?)

The Black Forest (or Baan Mora - although "Baan" is apparently not a literal translation for "black") would be a very dense and thick forest, like Lan Orethan, but much darker. The sunlight would hardly get through, and many dangerous bugs would live there. The Telvannis like this forest because it isolates their district from outsiders, along with the Dustlands and Boethiah's Spines. It is advisable to go by sea to get to the western part of the district. This forest is mainly composed of big purple mushrooms with big caps, and trees with bushy leaves. The only light that can be seen in this forest would be the one emitted by various plants or mushrooms... although some creatures would use lures to attract their prey. Some Telvannis also come to isolate themselves in this forest.

Feivelyn Art - An inspiration for the Black Forest.
 

Feivelyn Art - An inspiration of a possible border between the Spines and the Black Forest (we see a small entrance on the right, from where the ashes would have passed).