My guess is (this has been in there a while, and I've droned on about it before) is that there are a number of planets beyond Pluto, but they move in retrograde, not unlike most of the outer minor moons of Jupiter. A probe could be the key to finding out one way or another.
And that’s a guess I can’t find argument with. And wouldn’t it be cool to find more?
The bottom line will be funding for any such probes. It’s a pretty safe bet the desire to find out is there.