Hey yo,
do not despair! I may not always have time for stuff, but I remember being thoroughly frustrated about the lack of reviews on my TR showcase back in the day. (Thinking about it now, I just couldn't wait to get started. I bumped my thread way too soon)
First off: This has some really nice ideas, little things that impressed me, like those details in the village ruins or the little exterior cavern serving as a connection between two stylistically different areas. I love the cavern roots cracking open the stone floor in one spot. You are on the right track with these little gems!
However, looking at the whole picture, a lot of things don't make sense or should generally be avoided, if I may say so. I'll just list everything I noticed.
1)
I don't know man, I've never seen mountains that go like this. It just seems as if you were trying to create something to have a cool bridge and a huge waterfall nearby. These things sure are neat, but if they go too out-of-context and if shapes are just really over-the-top-unnatural, people won't believe in what you have created. If there was any RL reference to this that will prove me wrong, I'll be glad to take this back. You can leave this in your showcase file, but when working on actual claims for any project I'd rather see some realistic mountain scales.
2)
I see you used the unscripted colony activators. Good catch!
3)
Why is there so much snow left in just a couple of areas, while there's literally no snow at all all around?

I know snow doesn't always melt in regular patterns, but it seems to me like you should've included some more bits of snow here and there to make the (necessary, since there is no version without snow on top) intact building blend better with its surroundings.
4)
Eh. I don't really like flora bled into rocks like that; it doesn't look good. Plus, you can see through the bottom of the rock here (we call that "caspering", in case you aren't familiar with the terminology yet).
5)
This looks odd. Showing destruction and chaos is a very powerful way to create dynamic exteriors, but in this case, I wouldn't say it makes a lot of sense. If you meant to show a couple of partially burrowed planks, you'll need to accept that Morrowind can't show this in a convenient way without making it look too abrupt to be reasonable.
6)
So where does this pile of rubble come from? The bricks don't fit the house ruin, and the castle ruin's too far off.
7)
Ew. What colour is that shading? That needs to go. Always stick to grey or maybe green or brown tones. No neon-red or cyan-blue or anything weird like that.
8)
Waterfalls are cool... but they don't quite work like this. While it looks acceptable in the CS, you will be horrified in-game. Each activator piece has its own animation, and you need to separate the meshes through rocks and such to avoid the "flat plane with animation on top" look at all costs when there's a lot of the same waterfall activators right next to one another. Use small rocks (especially in places where the water starts falling over the edge) and split up the waterfall into smaller streams that come back together at the bottom of your mountain:

9)
You got plenty of rocks at the side of this hill, why not on top? I know vanilla exterior had this frequently, but at least the absence of rocks was somewhat consistent. Plus, that ice shard sticking out of the hill looks a tad odd... probably just my opinion, I'd be fine if you left it there.
10)
Do these really grow here? There're usually seen around the Azura Coast or the West Gash Region. I think quite a few experienced players would be surprised to see them here.
11)
This is a texture seam. It usually comes up if there's three or more different textures right next to each other... it should be avoided that these are actually visible to the player. You may cover them with rocks, though I'd refrain from using plants to cover these spots. It's usually hard to do with the irregular mesh layouts and will likely cause you to try to cover a seam with multiple trees or plants arranged in a rather linear way, which'll probably look more stupid than the actual seam.
12)
I don't know about mixing this texture with the rock texture directly, too much of a contrast. Consider making a transitional area using a snow/rock texture.
13)
Classical floaters:
14)
HOLY BALLS style-mixing. Generally, every region has its own objects tied to it. We should refrain from using any rocks or flora from other region types. For instance, I should not mix flora_bc_tree_10 with terrain_ashland_rock_05 unless I'm making a transition from the Bitter Coast to the Ashlands, which is rather unlikely to happen in a direct way.
Let me show you what you mixed here:
- in_mold_rock_ with Terrain_rock_BM_. A really bad-looking combination, if you ask me. Iirc, mold style was used round the Ascadian Isles or Molag Amur, but not in the Velothi mountains. If you want to use a cave set in this exterior, stick to in_BM_cave_ (or the ice cave set as you have done in the appropriate bits).
-
Ex_S_Smith_Shed right next to a small Hlaalu settlement. Again, very odd combination. Cavern planks looked ok with the colony stuff though!
- Ash and farm textures from other regions. Really stick to BM textures, farming doesn't make a whole lot of sense up in the mountains, anyway.
- flora_tree_ac_ with Solstheim trees. The latter trees are perfectly fine here, but _ac_ stands for Azura's Coast, very inappropriate. Same goes for the parasols.
- You're lucky that scathecraw is actually part of Tamriel Rebuilt's concept for the Velothi mountains, however, I'm unsure about having lava vents in the middle of your farm.
- Probably shouldn't be using Flora_grass_. Use Flora_BM_grass_ instead.
- Never seen flora_bush_01 in the Velothi mountains. You'd see such flora around the Thirr river or in the Ascadian Isles.
14)

I'm unsure about this thing. It doesn't look right in this place, but then again, I can't quite put my finger on what exactly is wrong.
Well, so much for the (constructive) criticism... Now let's see what you got right:
- Best roads I've seen in a first showcase. Excellent.
- Almost no rugged terrain/terrain spikes. Usually the biggest problem in showcases, I was relieved by the absence of such issues.
- Absolutely fantastic amount of creativity. The exterior caves are splendid and create an interesting bit of diversion. The dwemer ruin is beyond beautiful.
- The small details, like the skeleton in the ruined village and (as I'm sure I mentioned already) the cave roots coming through the floor of that ruined fort piece. Very intriguing way of combining entirely different statics. Keep it up!
You mostly need to sort out these style-mixings, man. Other than that, there's a lot of great stuff to be seen here!