Maybe an extension of Mx, Ms, Mr: Mixtress? (I suppose the "ess" is what makes it seem feminine)
Or just "Mx. Name", like how "Mr. Bond" can feel like a single term in the James Bond movies. (Mx. pronounced "mix" feels right here)
Let's see:
- Mistress
- Mister
The end should be changed in stead. You can do the X thing: Mistrex (or Mistrix to keep the "e" part pronounced as "ih").
Or change it another way: Mistro, Mistrez?
You can say fuck on the internet.