I think Master Assassins could be a cool edition to this mod, even though they where only in 6th edition at that same time skaven ONLY ever when't to 7th edition to technically Master Assassins would only be a single edition off and would give skaven a cool new lord choice,
Hell if ogrehsaggoths where put into this for a lord choice I personally see no reason for Master Assassins not to be.

Anyways fantastic mod and hope you like my suggestion .