It would be nice to be able to change the colors on cosmetic pieces. Diablo IV (as much as I hate having to draw comparisons) did a good job at it with its transmog system, which made matching pieces from different sets and creating nice-looking mogs pretty easy. Dyes could be added as penance rewards, since they’re so thin already, containing only a few items to actually unlock. A simple cloth>leather>metal (meaning a color for each material, each material a region to color) system would be pretty great.
Also, if I may add: Psyker cosmetics have a weighting problem. The tops are mostly too bulky, but the legs are too skinny. That’s the case for most cosmetics for the class, including premium. It looks very strange. Like they’re wearing someone else’s lower body. Looser pants alleviate the problem, but there’s only like 1 model in game (penance reward, I believe), with 3 variations, and it tends to clip with top pieces that have dangly or flowing bits.