AAAAAA I LOVE THESE THINGS SO MUCH. <3 A DeviantARTist named Esepibe hosted a contest with this as the theme a few years back ago, except she referred to it as "slow evolution." Either way, the contest yielded TONS of cool entries and thus lots of really neat ideas. During the contest, though, there were usually more than just one design per evolution. So for instance, there would have been two stages illustrated between Dratini and Dragonair, and two stages between Dragonair and Dragonite.

This person has a LOT of these on their tumblr, so I encourage you guys to check them ALL out! She even has some flavor text for each one. It'd be so cool to start an art project with this as the theme here on the forums <3