Pretty much this for me. The only third parties who would actually properly excite me would be Professor Layton and Megaman. But like Peacock said. Laytons chances aren't that good even if he has creator's interest. His demand just isn't big enough sadly. :/
Don't lose hope just yet. Let's consider the things in Layton's favor (ASIDE from him being popular, iconic, unique, etc). You'll have to forgive my lack of sources, since I'm on my phone and it's far too much effort to hunt them down now. You'll just have to trust me.
1. He is a cultural icon in Japan. In fact, in some SF4 parody that was done where the characters were replaced by popular Japanese icons, Layton was included. Mario and Sonic were not.
2. Akiro Hino, Layton's creator, has expressed interest in having Layton be playable in Smash Bros. I understand that this doesn't mean TOO much (Travis Touchdown's creator wants to see Travis in Smash and we all know that's not happening), however who knows what could happen if Hino goes out of his way to request Layton's appearance.
3. Layton is practically a Nintendo character despite being technically 3rd party. The only game that appeared off a Nintendo console was some bizarre mobile phone game that never left Japan. In fact, in celebration of some sort of anniversary, Nintendo posted a photo of a shelf filled with all of the iconic games from Nintendo's history. Along with all of the first party classics, Professor Layton and the Mysterious Village was included.
4. Sakurai is aware of the possibility of making Layton a playable character (unlike for Ridley in Brawl). On a Japanese talk show, Sakurai appeared with Hino and the host of the show even suggested that Layton should be in the next Smash in front of both of them. The seed at the very least has been planted.
Now all of this could very well mean nothing to his chances, since it's all pretty much up to Sakurai's whims. However, I think he has a hell of a lot going for him than most third parties.