Not every relationship that starts out vanilla necessarily stays that way. When i first started seeing my ex-fiance he was totally vanilla, although vaguely dominant insofar as he always chose position etc.
After we'd been sleeping together for a few weeks i casually suggested a little soft bondage to 'spice things up' and gradually drip-fed the bdsm into our relationship from there (just because i'm a sub doesn't mean i can't be a manipulative little girl....)
He is now a poly with (at last count) 3 subs to his name, and is into things that i wouldn't even contemplate. Sadly we didn't work out but we are still close, and i was proud to have him as my Dom - training can work both ways
That's not to say everyone has a hidden inner Dom(me) or sub, but it can certainly be worth a try if you think the other aspects of the relationship have potential