How to include references to subfolders

Hello,

I've been searching what's the best way to include a reference to a 
subfolder/subdirectory from a (secondary) remote repo into a subfolder 
of my (primary) repo.

Here I found some approaches:
https://stackoverflow.com/a/30386041/509369

Where the first 3 options don't seem to support sending changes back to 
the mainstream; and the 4rth is replicating the whole repo.

Ideally:
- The reference is stored somewhere in the (primary) repo so when 
someone clones it, the referenced folder (from the secondary repo) is 
also cloned.
- You can pull changes from the remote repo into the subfolder of your repo.
- You can make changes to the subfolder directly from your (primary) 
repo, and push them to the remote repo (of course depending on the 
privileges of the remote repo).
- Allow multiple remote subfolders.

Extra: it could be more complex, but what about single remote files?

I'm not sure if this is possible with the current set of git tools, if 
it can be added in a future or if it's just a no-sense question.

Thanks in advance!

-- 
Salut,
Yeray