I did a little math, and there are 35 Pokemon out of the 493 from Gens I-IV. That's around 7%. In comparison, Water, the most abundant type, has 93 Pokemon, or about 19%. So while I think the rarity of various types makes a bit of sense ecologically speaking (Dragon and Ghost are the rarest types, by the way), I do think they could add one or two new families in Gen V without going overboard. I like Pokabu and Reshiram, don't get me wrong. But Hihidaruma is pretty cool, and it's neither starter nor legendary, so I want to see what other Fire-types they could come up with for us. If they're as good as that ape, I think we would have some really good ones.