Similar to how Bowsers Fury has its Island RTAs I think Mystery House RTAs would be a cool addition to the Level leaderboard for category extensions as they do not belong inside of the base categories as those use IGT.

List
-Mystery House Melee
-Mystery House Mad Dash
-Mystery House House Throwdown
-Mystery House Claw Climb
-Mystery House Brawl
-Mystery House Marathon

While these are all the Mystery Houses its not possible for their to be a Itemless category as some mystery houses requires you to have items to progress. Although what I do think there should be is co-op for these Mystery Houses. The times are faster if you work in co-op compared to solo for certain mystery houses and I think it would be quite interesting.

The only Mystery house that requires an item as far as I know is you need a Fire Flower for Mystery House Marathon torch room and maybe a cat suit for one or two of the cat challenges also in Marathon. Itemless is probably possible as a sub category and some hand waving in Marathon.