1. We only need one Slime which is a star, and we have one. What's your point?
2. Nintendo put Roy to promote him. They won't put a third party for the purpose of making them more deserving of being in Brawl.
3. Personally, I think some bland character with a set of keys is a bit of a worse choice than a mascot which can use swords, staffs, magic, ride in a tank, etc.
4. It's NOT Nintendo's job to advertise hypothetical games. If they were going to advertise any games, why not - oh, I dunno... the more than half a dozen DQ games that have been or will be released for the DS and Wii? Slime can do 100% of the things Sora can do.
5. We can't assume this, though. The fact that KH fans might be on Wii doesn't mean we should assume it. There's no known market for KH.
And if you want a moveset for Slime...
B: Borg Slime - Puts on a robotic HMD and shoots a laser
Smash B: Blaze - Spits a ball of fire
Up B: Angel Slime - Flies into the air with angel wings
Down B: Burrow - Digs down into the ground and eventually pops up
Final Smash: King Slime - Transforms into a King Slime, a heavier, slower, and stronger character
Simple.