Leon's title would be retconned then since problem is next: in Pokemon Mega special we have seen how Earth looks and it looks fairly similiar to ours since regions are based on countries. That means we have a lots of countries to uncover. My opinion on countries which should be done.In Asia; SK,Turkey,Russia and India. Australia: well Australia and NZ. Europe:I would say Sweden,Germany,Spain,Greece and Italy.
Africa: Egypt ( IDK about others) maybe Madagscar.South America: Argentina and Brazil.
NA: Mexico, some parts of USA like LA, maybe Texas etc and Canada.
As you can see that's a lot like 20 or more(or less) regions to uncover. Now even if Leon is world's strongest where are Champions, gym leaders or E4's from those regions? They don't exist yet? Exactly. Even if PWC is world tournament it is not really, since there won't be anyone outside known regions(main and side). Anime can't make up how those Champions or gym leaders would look like, so they have to wait for GF to do that. Since we won't see those people battle we don't know if they are stronger than Leon( Champion of Germany could be stronger than Leon since he basically didn't exist in PJ) also Ash doesn't know that too since he would never battle those Champions, so Ash should battle with them. Not to mention that Ash will catch new Pokemon that will be really weak and he will train them since he just caught them and Pikachu varies in strength until the end so even if Ash goes to new region it will be from a scratch because of new Pokemon caught and Pikachu being inconsistent.