Paul seems to respect Brock

We know that Paul, just like Ash, traveled through Johto, Kanto and Hoenn. It would be cool if Paul also had lost to Brock in a gym battle back in his Kanto days. And that would be one of the reasons why he respects Brock as a trainer and person.

It would also create an interesting parallel with Ash, which also lost his first battle against Brock.