I don't know if they should... but they seem to be hinting at it... in magnezone's pokedex entry in sinnoh it talks about scientists trying to evolve magnezone.... do you think this sort of thing should happen? --- Warning! this post may cause severe laughter. viewer discretion is advised...... read it, it's funny!
I vote no. The limit needs to be somewhere, and three has always seemed reasonable. Plus, this far along in the series, there's how many third-stage Pokemon? Are we just going to go back and give them all evolutions? --- ....because shutup. R - Hyuck hyuck hyuck!
I would be more in favor of them going back and giving useless pokemon an evolution that made them useful before they went ahead and gave already strong pokemon an evolution that made them even more powerful.