I’ve been with my GF for a while and she’s by far the most submissive, sexually open girl I’ve ever had. She’s a real pleaser and hasn’t ever hesitated to do anything I’ve wanted. We’ve long passed the point where I would normally get resistance from other girls and I’m running out of ideas. I don’t want to miss opportunity. If you were in my shoes, what would you have her do?
Well, everything in steps because she may very well have a limit you don't... and you just haven't found it yet.
I'd suggest having her perform in an amateur night at a strip club, and if that goes over, ask her if she wants some lap dances. From there you can move to an MP or outcall if it seems right.
You could also try out the exhibitionism stuff in a safe way by posting pics with her face out of frame or otherwise obscured on one of the many amateur boards out there.
If you're looking for a 3-way, it's probably best (and I don't know this from experience, never having had an SO who was interested) to have your girlfriend find the other woman, and don't touch anyone but your SO the first time, and even then not without a long discussion.