Git Partial Fork, synced pull/merge

I want to fork a Git repository A that works with interface X, to my own repository B that works with interface Y.

To do this I would remove X-related files and create my own Y-related files, however I want to be able to pull changes to the core from repository A, in to my repository B, without pulling in X-related files (that I've already removed) and I want to be able to submit changes (i.e. pull request) I've made to the core, without that pull, removing X files from A or adding Y files to A.

From looking through similar questions on this site I've found things like rebasing, and selective merge, I'm not sure if this is actually what I want or not.

My main concern is keeping track of who made what changes, but I also would like to keep the relationship between the to projects if possible.

P.S. both projects are on github, if that provides any extra options

Solution

In the situation you describe, you cannot merge between A and B. You could instead create an in-between branch A':

Merge A into A' to obtain upstream changes.
Commit shared changes to A'.
Merge A' into B to apply these changes to your Y-specific code.
Commit Y-specific changes to B
Never merge B into A'.
If you have changes in B that you want to share with A, cherry-pick them into A'.
Request pulls to A from A' to share your changes upstream.

Other possible solutions include supporting both X and Y in both A and B, or splitting X and Y support into separate repositories.