Dolly
Steve and Tony have a happy relationship, practising a total power exchange for quite a long time now. And usually, Steve would say that nothing could be done to make it better, except maybe have SHIELD bothering them less.
At least not until Tony compares him to a doll Steve never heard of. Or, more likely, until Steve fell down the rabbid hole the internet could be.
Now he not only found a few new kinks, but also a new obsession - which he only has to admit to Tony.
Well, who could've known that a normal Ken Doll could open up so many options?