Models of solar system formation, based on current observations of exoplanets, is that we started off with maybe a hundred planets. They moved around, got ejected or fell into the sun.
So, dozens of planet-sized bodies got thrown out of the system -- several times the number that stabolized. Where did they go? "out there". But if all the other stars do the same thing, there would be as many coming as going.
I'm thinking that even if thrown out at escape velocity, there would be drag as it passed through other systems, and eventually it would hang out in an Oort cloud. Assuming such clouds basically reach to where the star stops dominating space, an object that started out between systems outside of the Oort radius of any star would be chaotic and fall toward one of them eventually. It would experience drag from the cloud, and eventually would not leave again after falling toward one.
So, I speculate that there are many planet-sized bodies out there. Pluto is certainly not the last largest!