I was really disappointed when I heard solid snake was going to be in the game. That destroys the whole gimmick of the game: Nintendo favorites beat the crap out of each-other. Not that snake won't be a good character, I just think it's straying a bit from what made most fans love the first two games. And, the banana peel isn't new. It was in melee, and probably the first game.
I'm fairly sure that Ridley (along with Diddy, and sonic) hasn't been confirmed yet as a cameo, let alone a full fledged character. I think what will happen is that Ridley will appear as an assist trophy. In case you don't know what those are, they are essentially poke-balls. The difference is only that they are not limited to poke-mon. I think, even without a completely Nintendo cast, it's shaping up to be a wonderful game that'll attract even more fans. I can't wait!