It’s too bad about having the NCFF masteries, if you took them out you’d have exactly enough mastery slots to put in the D2 masteries, though I understand why you wouldn’t want to do that at this point and ruin anyone with a character currently using masteries from NCFF.
As for single mastery mods that you could add, the only notable ones for me are two very old ones that never got updated with the expansions (which doesn’t seem to be an issue for you), the Engineer and Mystic Warrior. Fair bit of warning, they aren’t the most balanced, especially Engineer. There’s also the new Mastery Plus mod which adds an extra mastery that acts as an extension of your other mastery for a sort of single mastery build, but of course only works for the original GD masteries. Not released yet, but could be interesting when it is.