Many pokemon have had their shiny forms changed through the years. There's the most notable being Charizard going from a purple coloring to the now iconic black color. Arbok, Spinarak, Bellosom, and Ponyta are some other examples. I believe a fun easter egg for shiny pokemon who have had their coloring changed would have been to keep those original colors as they migrated to future titles.
An example is Swinub. The original color was blue, then it moved to a more pale green color, eventually settling on its now pastel green coloring. If you had caught a shiny swinub in gen 2, the blue color should have carried on into gen 4 games and beyond into the current games.
This would have been a fun way to encourage players to migrate their pokemon over to future titles, and now with the online capabilities, players could have shown off their purple Charizard, marking that they caught this mon all the way back in the gsc games and have carried this precious partner into the current games so many years after
by A-A-Ron2127