Since I already made Desert Region 2, I figured out what would be in the third one already. It will be put in Half-Life (I have a fair idea of mapping and world building) and it'll mainly take place in Desert Region, quite close to Black Mesa. Now, I won't reveal too much yet but the character Marat will actually talk with my recorded voice. I don't have Half-Life yet, but I will soon.
Watch this space.
EDIT: For those ones who loved Desert Region 1 and 2, I have some bad news. Desert Region 3 is planned to be the final one which continues along the "protecting Desert Region" storyline, I'm afraid. The other fully moddable games just won't have the desert sands, according to the whole storyline.
The future of characters like Marat is highly uncertain, I'm afraid. At least, not at the moment. So far, three mods are actually enough to conclude the storyline. Any other series emerging is uncertain as well.