The ability to make Cait's tail longer, either temporary for sex or permanent as an added combat tool. Probably for tease?
The ability to give transformative items to Brienne like with Cait. My apologies if this is already planned but not yet in because Brienne is brand new, but I wanted to mention it here in the event that it wasn't. (The ability to give companions transformative items in general would also be nice.)
Potentially the ability to shift companions around a few "states" with the proper items? Like for example you can give Cait a bunch of BoobGrowItem™ to give her hyper breasts, and then a bunch of BoobShrinkItem™ to get her back down to normal breasts, and so on and so forth.