I think they should have a few main rivals, but also have other recurring characters; that way all of the final competitions (Pokemon League, Grand Festival, etc.) aren't full of CoTDs. Nando, Conway, and Kenny weren't treated like actual rivals, but when the time came, they were there. We knew them and loved or hated them. Even if they appear in six episodes total, it'd be nice to have a few established characters there. Ash meets so many people who are also collecting badges and yet they don't make it to the League. You'd think a few of them would be there.
I just hope they do a little planning to accommodate the rivals. We can all agree this League is a rushed mess. Hopefully in Isshu, they go at their own pace, even if it means not ending right after the League. There's no reason to have it be Pokemon League -> Filler -> Pallet Town -> New Region. Even if it's not a mini-arc, they could still take 6 to 10 episodes to wrap things up and do some fan-service.