There is one hard limit my husband has: our relationship is to be between just the two of us. He means the obvious - neither of us is to be physically intimate with someone else. Yup - ok - that is how our marriage is, I considered it an absolute given anyhow.
He also means that neither of us is to become emotionally involved with someone else in the way that he and I are. That one is a little trickier - but it means that there should not be elements of a D/s dynamic or power exchange with anyone else. I have little opportunity for that to happen in any case, but i can see that it would be harder to identify or monitor. Also there are aspects to a D/s dynamic that have nothing to do with sex, so - how would that fit in? I think that's actually an interesting topic and I know very little about it, but again, no real opportunity, so i'm not going looking for hypothetical trouble.
Someone asked me the other day how i felt about playing/scening with another couple. She was mentally exploring scenarios and asking where i would be interested in fitting in, and where i thought i really could. What i would be interested in ended up being quite a few different scenarios. She set the scene, so to speak, for some fantasizing in directions I hadn't thought about before, and, well, it worked for me.
As to what I really could do - what would my husband be comfortable with is one question and what do i really think i could do or try is a very different one. In talking about this with my husband, it became clear that the lines can blur a little and that made us think about what means what to us. There is a whole host of activities that aren't in and of themselves erotic or sexual - spanking, clothespins, ropes, candles, etc... But between him and me, these are all very erotic, very sexual, and very arousing. I think that, even with someone else, there would need to be already, or there would be created, some kind of intimate emotional connection around these activities. But that part, I don't know, I just don't have the experience.
And what could i really do? what are my real, actual limits - I just don't know. There are obviously people out there who know how to do things we don't, people who know how to use techniques my husband doesn't. We have the rest of our lives to learn, and I expect we will have fun trying. But there are some things he is likely never going to want to try. Just because some of those things play into my fantasies, could I really participate, submit to them in reality? Along the same lines, playing intimately with another person/people is not on the menu, but could I really even if it were?
I have decided that there are things that are very strong fantasies precisely because they are not going to become reality. The fact that they are purely hypothetical is what makes them safe to consider. If they became, even potentially, real - I'm not at all sure I would feel the same way about them. I suppose that's the part that makes fantasies just that.