The idea of another large planet in our solar system is exciting but could someone more knowledgable clear something up? Do astronomers expect that there might be more large undiscovered planets or is the prediction of this most recent one considered a fluke?

I'm not an astronomer, so don't take this as gospel, but I believe that large undiscovered planets are expected to be rare. Mainly because they would have had to form in the inner solar system (where all the matter was) and then somehow flung into the outer reaches of the solar system (perhaps via a major collision).

Relevant XKCD: https://xkcd.com/1633/