I added a unit to TD that works like the Sputnik/Prospector in RA3 or the Surveyor/Emissary/Explorer in Tiberium Wars and lets you expand your base.
Its made as an example of how to do it and if you wanted it in your mod you would probably want to make new graphics that aren't so big (right now its using the graphics from the MCV and construction yard from RA since I don't have the skills to make new art)

You can download it from https://drive.google.com/file/d/11OoO-mZnX90FxRm6dvqmqskYim-OjHw-/view?usp=sharing (its too big to attach as a forum post)

Source code changes I made: (not all are necessary or even desirable depending on what you want out of this)
1.Change the price of a number of things in bdata.cpp to make testing easier.
2.Add new "deployed expansion unit" structure to bdata.cpp
3.Change building.cpp so the new building can be undeployed back into a unit.
4.Change building.cpp so you get an engineer like you do with the regular construction yard.
5.Change building.cpp so the new building will handle repair stuff just like the regular construction yard.
6.Change building.cpp so it will recognize the new building when testing for building placement and let it deploy anywhere just like the MCV can.
7.Change conquer.cpp so that the h key will center about the new building and vehicle like it does for the regular construction yard and MCV.
8.Add the new build and unit to defines.h
9.Change display.cpp so that the new unit will be excluded when you do a drag-select the same way an MCV is.
10.Change init.cpp to load mod.mix
11.Change sidebar.cpp and unit.cpp so the new unit uses the same special-case remap logic for NOD as the harvester and MCV.
12.Change the price of a number of things in udata.cpp to make testing easier.
13.Add the new unit to udata.cpp
14.Change unit.cpp so the new vehicle will be picked up properly as deployable and will properly deploy into the new building.

Other changes made:
1.Create mod.mix containing the legacy graphics (the legacy graphics are just copies of the shp files from RA, if you use legacy mode they will look out of place because the color palettes don't match)
2.Create zips for the remastered graphics (these are just copies of the remaster graphics from RA)
3.Update cncbuildables.xml so the new unit/structure are in there
4.Update td_structures.xml and td_units.xml for the new graphics

I intend to do some more experiments to see if other unit/structure/etc ideas from later C&C games are possible to add to the remasters.