Okay, so there seems to be a heated debate between Mega Man, Layton, and Square Enix. I'll pitch in my thoughts now.
Assuming the likely scenario that only one new third-party character gets in (if we even get any), I feel that it will be either Mega Man or Layton. I'm currently saying Mega Man because Sakurai has been aware of the fan base for Mega Man to be in a Smash game ever since Pre-Melee and Sakurai does seem to be open to the possibility of a Capcom character. Capcom has also expressed interest for him to be in Smash (and isn't there something about Capcom developing Mega Man for Brawl, but negotiation fell through? Correct me if I'm wrong on this.). However, I still feel as Layton is possible as an alternate assuming that negotiation for Mega Man does fall through as the president of Level-5 did express interest in putting him in Smash (which is something significant to note) and he does seem to be decently popular in Japan as a choice. Layton is a character that I feel has serious potential to gain a strong fan base to become a playable character and I can see the president of Level-5 pushing for Layton's inclusion. While I'm not particularly interested in Layton, I think he works.
I'm also not sure if Sakurai considers strong third-party relationship with Nintendo and another company as a reason to include third-parties. It is true that Nintendo wants to establish better relationships with third-parties, but there's no proof that this is going to carry on to Smash, because as much as we don't like to admit it (or like to admit if you don't like third-parties in Smash), the possibility does remain that Smash 4 do away with them and I think that strong popularity or interest by a big name in gaming like Kojima or a president of a company will help more than just strong corporate relationship that may mean absolutely nothing to Smash in the long term.
As for Square Enix getting a rep; I don't have much hope for it and if it does get one, I'm worried that it could end up being Cloud (assuming that a 3DS Final Fantasy VII does happen), which I would be very displeased with seeing. As for Slime, whom I think would be great in Smash, I have a very bad feeling about him. Starphoenix have indeed given great reasons for why Slime should be in Smash and how Slime could fit in well, but the question isn't that can he fit, but this; does Sakurai think Slime can fit in?
On the one hand, Slime has a lot more to work with than Mr. Saturn, but he still has no limbs, which was why Mr. Saturn never got in. Plus he's third-party and those are extremely difficult to fit in, which could give Sakurai less of an incentive to consider Slime in.
But overall, I'm just hoping that Sakurai is relatively open to third-parties. Even thought they are the least necessary part of a roster and one that we can go without, after Sonic and Snake were such enjoyable inclusions in Brawl and the roster bound to expand to 49+ characters, it's hard to look back (especially with Mega Man being my #1 most wanted character
).