For spriters: Honestly speaking Gym Leaders and Elite Four should be custom made. Revamps/Re-colors can work for trainers, but the GL and E4 should be custom.
Storyline writers: Since we obviously started to think of stuff for GL's we should think of what Types they're going to be first so that spriters can work on their appearances. I was suggesting:
GL #1: Male, Boy, Bug Trainer
GL #2: Male, Teenaged, Flying Trainer
GL #3: Female, Young Lady, Grass
GL #4: Female, Somewhat Old Lady, Water Trainer
GL #5: Male, 20's, Fire Trainer
GL #6: Male, Somewhat Old Man, Ghost Trainer
GL #7: Female, Teenaged, Fighting Trainer
GL #8: Male, 20's, Dark Trainer
E4 #1: Male, 30's, Rock Trainer
E4 #2: Female, Teenaged, Steel Trainer
E4 #3: Female, 20's, Ice Trainer
E4 #4: Male, 30's, Psychic Trainer
Champion
I couldn't think of anything for this one. But maybe (s)he doesn't have to have a type and just use random pokemon from the region.
Anything here is free for change, but I'd thought it'd help Spriters/Map makers plan how they want to be make certain things.