On a more positive note, his death could inspire other trainers to be more like him. Young trainers might see his selfless sacrifice and be motivated to become better, more ethical trainers. They could strive to form deeper bonds with their Pokémon, just as Ash did, in his memory.
The Pokémon world in the fanfiction could be thrown into chaos. Ash was known as a great trainer and his presence had a certain influence on the balance of power. With his death, other trainers might start scrambling for power or resources, thinking there's a void to be filled. Also, the regions he visited might experience a decline in tourism as many trainers and fans were attracted by Ash's adventures there.