I've never been a huge fan of Raichu so I would love another evolution that resembled Pikachu more. My guess is that the stone causes a mutation which is why Raichu looks so different from his pre evolutions. So this should be a natural evolution through leveling up. They could even make a story to explain why they couldn't evolve before. Something like "Pikachu from all corners of the world have come here to train and evolve without the need of a stone." Perhaps make a special location where it can evolve like Leafeon and Glaceon.
I know this will never happen but a guy can dream.