Which pokemon could do with a type change?

[Chaos Rush]

I always thought since Venusaur and Charizard have a second type, that Blastoise should have one too. For some reason I always saw Blastoise being Water/Dark as fitting, even though there is nothing particularly "Dark" about it, you can say the same for any other Pokemon. (Starmie is Psychic!?!?!?)