There are several reasons why I feel this should not and will not happen. I will name a couple (perhaps more if I feel like it).
1. The character does not originate from a video game.
2. His popularity has waned even among 11 year-olds in the English-speaking world, and he will have rather limited appeal for the gaming population in Japan, the primary audience for which the Smash Bros. series is made.
The fact is, absolutely no characters from American childrens' animation are going to appear. No characters from that faux-anime piece of rubbish, Avatar: The Last Airbender, are going to appear in Brawl. Don't hold out for Kim Possible or the Rugrats, they are not viable choices. Fans would lynch Masahiro Sakurai for making a mockery of his own great concept, and I don't think he's too keen on being lynched.
I'm sorry if this seemed too harsh, but this is the first thread of this nature I've actually gone and replied to and wanted to get some of this off my chest.