Unsure if this is already an option or not. But a feature to track missing transmog appearances from your class sets would be nice. I'm not the kind of person to chase down every piece of transmog. But I'd like to fill out the 'Sets' tab. Perhaps additional options for difficulties ranging from LFR -> Mythic etc. Unsure if Blizzard API supports just pulling this transmog data?

Should be possible with C_TransmogSets.GetAllSets()
wow.tools also has ItemSet table which effectively has all item sets in game I guess

There are currently no plans to separately list sets, there are other addons that can be used to track these missing appearances, which ATT can supplement with info on where/how to gather these.