I think Eevee was first introduced to add on a type to counter your rival's in Pokemon Red and Blue. So you could get a Jolteon to face Blastoise, stuff like that. Though, now it's become an advertisement for different types of evolution - stones, night/day and level up in places. It wouldn't surprise me if in the future a dragon one was made to show a new type of evolution method. Trading with a certain ribbon or something.