Hey, I am just going to say this from a future buyer of the games point of view. This of course will depend on the price, but one of the reasons I am Okay paying $60 for SSB4 is because of the content. There are over 50 characters to all master and such, and lots of content. If you are going to sell this game with only 8 characters, it isn't really all that much, I would recommend at least doing 12 characters. I know that making a character roster that is balanced and equal is VERY hard to do, but If you add more characters, that may make the meta-game better, more choices and cooler match-ups, etc... Maybe do a mix of each element?
Rock + Fire = Lava / Water + rock = Ice / Ground + Water = Steel? Don't take this offensively, just a suggestion.