A great plot for 'pokemon red and green fanfiction' could be about a trainer who discovers a hidden, ancient Pokemon that has been forgotten by time. This Pokemon has unique abilities that no one has ever seen before. The trainer then has to protect this Pokemon from evil organizations who want to use it for their own malicious purposes. This would involve lots of battles, strategic planning, and character development as the trainer grows stronger and more determined to safeguard the new Pokemon.
Well, you could write a fanfiction where the main character is a descendant of one of the original Pokemon Red or Green game characters. They inherit a special Pokemon from their ancestor that has a bond with their family. But there's a curse on this Pokemon that the main character has to figure out how to break. Along the way, they meet different trainers and Gym Leaders from the original games who either help or hinder their journey.