About a decade ago when I was more into roleplay what a mistress wore was relatively important but not so much of late. In fact if she is very attractive and there is good rapport I make it known after a few sessions that I am OK with her wearing street clothes during the session.

I'm unreasonably proud of myself when I have a unique answer ... or at least, think I do. For me, I like the overall context of a session to be "regular girl uses the power of female sexuality to dominate and humiliate me." To get that context, I prefer, for example, an apartment or hotel room over a formal dungeon. Not easy to suspend disbelief that she's a professional dominatrix, if we're in a fully-equipped dungeon.

Likewise, I like her to look as "civilian woman" as possible. My guidance to her is usually along the lines of: Within whatever your personal boundaries are, just raid your lingerie drawer... something like boyshorts and a tight clingy cotton top, or some yoga pants, or anything along those lines would be awesome... please no leather, no latex, no anything costume-y or dominatrix-y

Sometimes i think about roleplay and see Mistress in a teacher outfit