It sounds like your main issue is trying to figure out which of these is the future for them - if your boyfriend dates this person but there isn't a BDSM dynamic of any sort you are fine? If your boyfriend wants to explore any kink with her however, it is not fine and you will stop dating him?
My husband has dated a LOT I mean a LOT of kinky people, ones who were slaves, ones who were dominant, ones who did things that made him rather queasy, even ones that make ME queasy which is harder to do - but his dealings with them were always vanilla, as he isn't kinky, and he did not let what they chose to do with other partners keep him from liking them and dating them, as long as their dealings with him did not try to impose things on him they weren't comfortable with.
It really sounds like you may be coloring her with your past a lot. If he decided he wanted to date a submissive and be dominant to them, would you still be uncomfortable with it? I am just curious if it's just your past, or power dynamics in general, or maybe that you find a man who will let a woman dominate him to be weak in some way? If you really think he would let some new partner sway him into breaking up with you, you might have some other stuff you need to be communicating about, as it seems this also might be complicated by feeling you are as important as you like to be, if you think he'd let somebody tell him to break up with you.
Happiness will never come to those who fail to appreciate what they already have.