Each game should have at least one trainer with a shiny Pokemon. Gen 2 has the Shiny Gyarados just to show off the new mechanic to players who would otherwise never see it, but FRLG also had a few NPC trainers with shinys in the battle tower.
I think each game should have at least an easter egg hard-to-reach trainer with a shiny on their team. It would be kind of fun to run into a random bug catcher with a shiny metapod or an ace trainer near the end-game with a shiny stage 2 starter
Thoughts?