Some more suggestions:
Less lewd (relatively speaking) version of the braless Negligee.
Same color options.
Same Location.
Still transparent.
Similar price.
Still transparent but visually covers up more - the bra section is intact and the bottom part extends below the characters crotch.
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Exercise shirt
The rim of the shirt matches the Bloomers with the res of the shirt white.
Alternate graphic if worn with bloomers - instead of the shirt being layered over the bloomers is layered under to give the look of being tucked in.
Standard graphic is the same as the white starting shirt with colored trim.
I believe Indev mentioned somewhere that they wanted clothing to be fashion choice instead of a attribute choice for the most part so a conditional alternate art should not be to much of a stretch
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Consensual head petting cassie.
With the talk of cassie being a pet and another character encountered as a random fight pretty much accepting the role of cassie's pet this idea might be an acceptable addition.
After clearing cassie's route renovating the basement into the private dungeon the character could persuade cassie to try it out.
Among the options could be one 'treat her like a pet' - she does not not like the idea of being a pet but by the time the relationship has advanced this far she might be open to doing so of her own free will, even if some persuasion is required (A speech check).
Among those pet play options could be the option to 'pet her head and call her a good kitty' which causes her to start purring ... before she realizes what she is doing and says the safe word stopping it early.
After a period of time passes (a fade to black sort of thing) the next time you see her you have the option to talk about it at which point she will shyly admit (blushing, looking away) she enjoyed the player character doing that and would not mind them doing it in the future.
Doing so outside of time in the dungeon causes her to purr and changes here expression to a content one, until something changes.
More of an little bonus if advancing cassie's route that far.