Re-creating the film grain look isn’t that difficult, it’s the color matching that I’ve found to be really tough because each scene (and sometimes each shot within a scene) requires a different color grade. The release of the Fellowship extended 35mm rendered this moot but I’ll probably try and create a ROTK extended hybrid now.
If you do a ROTK hybrid, you might want to make it a version where you add only the scenes that matter. Keep Saruman, don’t keep farting Gimli.
Keep the ghosts, but don’t keep the lame pirates.
Just my two cents.