Somebody sent me a PM asking me to make some analyses for Yoshi and Wario characters. I did do just that, and here they are:
Kamek: Kamek was the main antagonist of Yoshi's Island and Yoshi's Island DS (and he also appeared in Mario & Luigi: Partners in Time). He is a magikoopa that continually plagues the Yoshis and Baby Mario with super-sized versions enemies you come across in both games. However, Kamek has not appeared specficially outside those three games (other than maybe Yoshi's Touch&Go, which I haven't played). Furthermore, it is usually Baby Bowser you fight at the end of the Yoshi games. However, unlike Baby Bowser, Kamek bears no semblance to any other probable character, and would offer a unique moveset. The main problem with Kamek is that he hails from the Yoshi series which is pretty much a Mario spinoff series. The most I can see Kamek being is an Assist Trophy.
Baby Bowser: Baby Bowser was the final boss and antagonist of several Yoshi games, such as Yoshi's Island, Yoshi's Story, and Yoshi's Island DS. However, Baby Bowser has a big problem, and that problem is that he is too similar to Bowser Jr., who has become an important character in the main Mario series. Between Bowser Jr. and Baby Bowser, Bowser Jr. is much more likely to get in as a playable character. However, I do expect Baby Bowser to show up as a trophy, which he appeared as in Melee.
Baby Mario Bros.: This is an idea that I see get thrown around sometimes. I see people suggest that Baby versions of the Mario Bros. work as an Ice Climbers-like character, and an unique moveset can probably me made for them just based offf of Mario and Lugi: Partners in Time. However, they suffer from hailing from Yoshi series, and for being younger versions of two characters that are already playable, in which case Sakurai and his team might opt for a more unique character. I do expect at least Baby Mario to show in the form of a regular trophy, which he did in Melee.
Jimmy T.: Jimmy T. is Wario's best friend in the WarioWare series. He loves to dance and a has a big blue 'fro. I imagine that if he were to get in, he would probably have his moveset consisting of dance moves. However, this would involve Sakurai's team having to create a unique moveset for him, something that they will probably only reserve for important characters from important franchises. The Wario series will probably only have one representative, which is obviously Wario. However, I do expect Jimmy T. to show up in some form, whether it be an Assist Trophy, a regular trophy, or a sticker.
Mona: Mona is another one of Wario's friends, and she often switches jobs between games. Though Mona is quite important to the Wario series, she also suffers from the same condition as Jimmy T., in that Sakurai's team would have to make a unique moveset for her (meaning none of her moves would derive from the game she has been in) in order to make her a playable character. However, like with Jimmy T., I do expect her to show up in some form, whether it be an Assist Trophy, a regular trophy, or a sticker.
Ashley: Ashley is a rather recent addition to the WarioWare crew. I don't think she is as iconic or important as Jimmy T. or Mona, though she does have a few devout fans on this site. While her moveset could possibly consist of dark magic, I don't think Sakurai's team would go through the trouble of making a unique moveset for a minor character from a minor franchise. Her theme being in Brawl has made some to believe that she will show up as a playable character, however, I do not think this means much. I do expect Ashley to show up as either an Assist Trophy, a regular trophy, or a sticker.