When New Horizons launched, its mission in the public imagination was to complete the set of the nine planets. No doubt this helped the mission get funded, and I respect that.
By the time it arrived, though, we had accumulated enough evidence to establish that Pluto is not best thought of as one of the “major planets” of our star system. We now think there are only 8 of those, or as I prefer to think of them, 4 “terrestrial planets” and 4 “gas giant planets” (with their accompanying moon-systems, containing more than a dozen major worlds among them):
Now, Pluto is better seen as the first Kuiper Belt Object to be explored. And thus it is the beginning of a whole NEW set of objects in our solar system. There are a lot of small worlds out there in the space beyond Neptune. The Kuiper Belt is like a second, icier asteroid belt around our Sun. And there are scattered worlds beyond those (collectively called “Trans-Neptunian Objects”, which include Kuiper Belt Objects, Scattered Kuiper Belt Objects, and more — we’re just beginning to learn about this part of the solar system, because we’re only now getting sensitive enough telescopes to see these objects).
Eris is almost exactly the same diameter as Pluto and quite a bit heavier. It is currently about three-times further from the Sun, though in the middle of the 23rd century it will swing back to a distance similar to Pluto’s in its 558 year orbit.
Sedna is currently just a little bit closer, and will be making its closest approach late in this century — only to swing back out into the dark for over 11,000 years, in a ellipse that carries it nearly to 1000 AU, where it spends most of its time.
There may be other worlds out there. Some may even be larger than these. Certainly there are many smaller ones.
There will always be more to explore.