Yes I can confirm this is true, since this post was made, this map has increased in size and many areas filled in now. It is percieved TR aim to, one day, fill in these missing pieces. Think of them as areas nobody has inhereted yet, wild country. Someone must adopt an area and devlop it in order to add this. But don't go exploring them, you will probably die from MW's automatic repositioning of the character. 

If there is any issue here, it is that these areas should be walled off or the player should be prevented from entering, and the map should have \\\ lines through the undeveloped areas (or grayed out). I would task someone to do this, perhaps a side mod, which would clear much confusion from the new users!

TR is quite impressive. If you are ending up here and also have other problems, the most common are these two:

1) Make sure you have all the data files (under "Data Files" in the launcher) checked so they can load, not usually something you miss

2) More often than not, players either forget to get the TR_Data pack which is separate, but you also have to add a few lines to your Morrowind.ini file, under "[Archives]" you will need to add lines to include the .bsa data files. Here is an example of how the section may look after the edit:

[Archives]
Archive 0=Tribunal.bsa
Archive 1=Bloodmoon.bsa
Archive 2=PC_Data.bsa
Archive 3=TR_Data.bsa
Archive 4=Sky_Data.bsa

Of course, your morrowind.ini file may differ. Just be sure you don't delete anything in there and you will be fine. If you mess up your morrowind.ini beyond repair, you'll have to delete it and it will be regenerated, but you will lose your custom data and have to put it all back in. For this reason, I suggest making a backup of any file or folder that you intend to change. It is better to have it and not need it, than need it and not have it.

If you don't "register" the .bsa files by inserting them, you will end up with untextured landscape and a whole lot of the dreaded yellow triangles with /!\ in them! This will be accompanied by "Missing Texture" dialogs both in-game and also generic ones (from the operating system).  

If you don't "checkmark" the files in "Data Files" from the launcher, you will not get any part of the mod at all, for those are the ones that modify the game (of course).

If you are worried about permanantly changing the data files tree, you have two options:

1) Make backups

2) Use a manager of some sort

Managers can be things like Wrye (recommended for heavy modders) or git (for programmers experienced with git only). I personally prefer git because I can roll back to whenver I changed something, so long as I had committed it, with little overhead. You MUST learn how to use git properly or else you could do damage. Wrye (Mash) is easier, and does a lot of that for you, but might take up more space. The upside to that is that it is built specificly for Morrowind modding and does checking at a very detailed level.

You can find these at Morrowind Nexus.

You can find git just by searching github or the original git websites in a search engine.