My opinions on these ones specifically (I left a few out just because I wanna give more than just "they would work" or "they wouldn't work)
Mickey: Wouldn't be in mainly because I know for a fact that Disney would say no. Also the majority fans would probably hate it. Sora from kingdom hearts would have a better chance of being in it, but even that is a slim chance.
Cecil- It's a possibility, since he was on a nintendo console first.
Cloud- No chance. He was never on a Nintendo console. Every character that has appeared in a super smash bros game has appeared on at least one nintendo console. Sure in theory he might be fun, but it would make no sense to have a character that has no association with Nintendo outside of Kingdom Hearts CoM where he had such a small role.
Bowser Jr.- Nintendo could have fun with him. He could pilot a little robot with a clown face and his final smash could be that he summons an airship that launches canons all across the stage.
Little Mac: Not enough moves to be a usable character. he's better as an assist trophy.
King K Rool: Could be interesting just as long as he isnt a rehash of DeeDeeDee. He has a gun he could use, boxing gloves, teeth, etc. his final smash could be that huge effing lazer tower boat thing he lived in, in DK64.
Roy: Honestly, I'd prefer him over Marth. I just hope that he, marth AND ike aren't all in the next one. Clones. Clones EVERYWHERE.
Mewtwo: I don't see why they took him out of Brawl. Lucario is cool and all, but Mewtwo is actually a legendary. He better not be replaced with Zoroark.
Krystal- She could work. I would prefer her over Falco and Wolf. Clone mania. Since she has her staff and psychic powers, she could bring a cool style of fighting that isn't just a rip off of Fox. Just as long has her final smash isn't the Landmaster. It would be funny if like she rides up on the Galdon and just has it eat everyone. Insta-death.
Paper Mario- Now this I can get behind. Paper Mario has a LOT of attacks that could be used in brawl. I don't know what they would do for a final smash, but I'm sure it could be fun.
Mr. Minecraft: Again. Minecraft. Not a Nintendo game. Also, I don't think that Minecraft could do anything other than a B attack and punches. Sure the final smash might be like a creeper or something, but still that wouldn't make much sense since the player doesn't control the creepers.
Ridley- He's too big and he can fly. He's much better as a boss.
Sylux- It could work, just as long as it's abilities aren't just a clone of Samus' moveset.
Mii: Now this is something that I kind of wanted with Brawl. You bring in your Mii and customize their moveset. I think that would be interesting. Or even a character creation mode in general could be fun.
Some characters that I think might work:
Earthworm Jim- He has enough attacks to work and his final smash could be the Barn Burner Gun or the Digging Gun (although the Barn Burner Gun would work best since in the games, it wipes out all enemies on the screen.
Daisy- Now this one is definitely a stretch, but it might work if they make her design different enough from Peach's. Her moves could reflect her spunkier nature and could be more sports and martial arts oriented than Peach. Also in Mario golf toadstool tour, she was one of the strongest characters in the game (beaten out only by Bowser and DK) so her physical strength in Brawl could also reflect on that. Instead of a dress, she could wear a much more casual outfit (like shorts or something) to separate her from Peach's design.
Shadow- Okay as much as I hate to say it, hedgehogs with guns could be fun. Also his final Smash
could be chaos control or chaos blast depending on how much he has been hurt or how much he has other others or something. Also he has moves from Sonic Battle that he could use. Although you know who I'd like more than Shadow?:
Robotnik: He could be used in a manner similar to that of Bowser Jr that I mentioned above. And, I mean, you got Bowser. You got Ganon. You got DeeDeeDee. WHY NOT ROBOTNIK?
That could be good for both game play AND story purposes. (it would be cooler if it was SATAM Robotnik, but that would make no sense since SATAM's Robotnik never been on a console).
DRAGONITE- YOU KNOW WHY!!
Mega man Trigger (or any megaman character that appeared on a Nintendo Console, but specifically Trigger)- He has a wide range of abilities, the shining laser could be his final smash. Also MEGA MAN LEGENDS 3.
Scott Pilgrim- Okay, this one is a bit hypocritical of me, regarding the never been on a nintendo console thing, but if he was able to be in it, he has plenty of abilities to use (specifically in the game) and it would be cool to see him duke it out with some of the video game all stars. Scott Pilgrim vs Nintendo. Also his final smash could be a sex bob-omb (or shatter band) concert or something. Yeah.
Magnus Gallant (from Ogre Battle 64)- He could handle like the sword characters, but he can also use magic. He'd be pretty fun to use. And his final smash could be the summoning of his entire revolutionary army.
Any of the main characters from Sin and Punishment- Awesome. Just saying.
As for assist trophies. I could do without those. I never use them. I don't enable any item except for the Smash Ball.