In fanfiction, if Ash became an Elite Four, he would likely have a much greater influence in the Pokemon world. His skills and experiences would be highly respected. He could use his unique bond with his Pokemon to inspire other trainers. Also, he might be involved in important decisions regarding Pokemon battles and training regulations.
Yes, there are. For example, some rom hacks add new regions, characters, and plotlines. You can find them on various rom hacking forums. Just be sure to check the legality and safety of using such hacks in your area.
One common theme is their adventure. Fans often write about May and Brendan going on new quests in the Hoenn region, encountering rare Pokemon and facing new challenges.
Perhaps a fanfic where May and Brendan are chosen to participate in a special tournament in the ORAS region. But there's a catch - they have to train a Pokemon that they are not familiar with. This forces them to step out of their comfort zones and develop new training techniques. Along the way, they also deal with the pressure of the tournament and the expectations of their fans.
For May, you could develop her character by showing her struggle with more complex Pokemon battles. Maybe she has to face a series of tough opponents that make her question her training methods. For Brendan, he could be developed by having him interact more with different types of Pokemon, learning about their unique behaviors and needs.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Pokemon fanfictions, including those featuring May and Brendan from Pokemon ORAS.
Since it's fanfiction, the characters could be anyone the author creates. They might be original characters inspired by the Naruto world, or perhaps they are existing characters given new roles to form the elite four.
It could be that the producers of 24 Oras 2023 had a different vision or casting choices that didn't include Mang Tani. Sometimes these decisions are based on various factors that aren't made public.