The Cloud reveal opened the floodgates to such extreme wishful thinking. The community needs to temper their expectations. Almost every top five or top ten most wanted list consists of a majority of third party newcomers. Smash Bros. is still a celebration of Nintendo. The third party characters that appear are considered guests for a reason. The outside characters that do appear are from an exclusive, elite list of iconic A-List guest characters.
With that said, even characters that fit this criteria are still sort of exempt, due to various reasons. For example, Scorpion. When Sakurai envisions a newcomer, he represents them as close to the source material as possible. Scorpion, and his game, are known for hyper violence and gore. If Scorpion was added to Smash Bros., he would not reflect his source material, since he would need to be drastically toned down. He has appeared cartoony and toned down before, but that is irrelevant, since we are discussing his source material. Snake wasn't exactly toned down either, Sakurai only removed his guns but he still retained his espionage persona, explosives and martial arts combat, which more closely represents his source material of a special ops stealth soldier.
Also, when looking at every third party character ever added to the game, there is a clear similarity. All of the newcomers have been ultra iconic characters from Japanese companies. Sakurai is hunting for icons, and has said in the past he isn't going to add third party characters so "willy nilly" and isn't adding them for the sake of adding them. Third party characters from western companies don't seem to be a priority, it's appears it's easier to discuss terms with Japanese companies.
Hopefully, I'm wrong about the last point, since I would love Banjo Kazooie, but characters like Scorpion and Master Chief are highly unlikely. Characters from much less popular Japanese franchises are also more unlikely than those from more prominent franchises. Realize that Takamaru, a first party Nintendo character, was turned down because of unfamiliarity overseas, so why add a third party character who has a similar degree of unfamiliarity here in the states, such as many Atlus characters.