The most obvious idea to me is to make the Champion Tournament that Cynthia mentioned.
"Every year, the Champions from all regions meet and battle to see who is the greatest" etc.
An adult Red and Blue could easily be opponents in said tournament. Heck, imagine doing a double battle against them.
OR GF can go the subtle route, and have some sort of museum of famous trainers. Then they could paste Red's sprite from the Gameboy, and call it a "statue of a legendary trainer from Pallet Town." --- "Love, hope, faith. The greatest of these: love." Check out my webcomic! http://www.drunkduck.com/La_Vie_a_la_Mode