I speculate Solid Snake because SSF2 have a Metal Gear Solid stage, so why not a character too?

Also, I remember Snake being considered to enter the game to the point of having even his own trailer years ago.

I guess that this may be more of a guesswork post seeing as I'm new to how speculah goes 'round here, but I thought I'd give a few of my thoughts.

First things first, Luffy. I already know the general stance on another anime character, but I've been watching enough One Piece and playing enough J Stars to see the immense potential in the rubber star of the current best selling manga of all time. All those stretch physics and gear changes would make for a really fun moveset, and are looking a bit uneven in their little corner.
Well dude, I can pretty much say that Luffy has a very high chance of getting in because of three reasons. 1.)He is heavily requested by fans. 2.)Goku, Ichigo, & Naruto are already in the game. 3.)Luffy is always the center of attention on these forums whenever people think of a character.

On another note, I think Snake would be a great potential new character, assuming Konami doesn't come in and throw Cease and Desists everywhere.
With Shadow Moses Island in here, I highly think Snake is a guarantee. The reason why he is probably not in any of the demos is because the Devs might be saving him as an unlockable character for the final version of the game as it seems like they could be mirroring Brawl.

Other than that, I'd say Takamaru would be a great newcomer to showcase what the devs are capable of on a basis of creating a mostly original moveset and design.
With the Devs adding lesser-known Nintendo characters like Chibi-Robo and Isaac into the fray, they could really dig deep into the Nintendo universe.

I'm also still holding out for Chrono, seeing that now that Cloud is an official Smash character his chances for Flash have kinda dropped off.
As I stated with Snake, I think the Devs are holding them for unlockable character statuses. Plus, they were both unlockable in SSF1, which could mean something.

Do you think the ssf2 devs would consider putting in characters from one piece, splatoon and maybe even look at super smash bros. crusade's roster for more added characters?

Sup. I'm new.
Anyway I speculate these characters:
Ike YES!
King.K Rool We need more villains McLeodGaming!!!
Knuckles and/or Shadow Both of these dudes were in SSF1, so I think both of them would make the cut.
Hoenn Pokemon Trainer (Without the stupid stamina system) Is this Red?
Monkey D. Luffy Dude, you are the man!! #LuffySSF2!!!
King Dedede Smash Vet.
Cloud Strife Now a Smash Vet.
Ganondorf Another villain, so yes!
Snake Shadow Moses Island is in the game, so I think he will be unlockable.
Zero I would love for him to return.
Pit Smash Vet.
Little Mac My SSB4 main!!
Waluigi Eh. No. But this is who you think will get in, so good luck to him.
Villager Smash Vet.
DHD (iffy on this one) He better be in because not only is Duck Hunt one of my favorite games, but it is also my third main in Smash 4 behind Little Mac and Rosalina.
Shovel Knight (same as DHD) You might not be aware of this, but this game's roster was finalized in March 2013, so Shovel Knight will not make the initial cut.
Well that's my list.

Goku2500: In terms of speculating who will get in, here are my picks...

Pac-Man (Because of the APEX 2015 leak) True Also because he is one of the biggest gaming icons ever.
R.O.B. (Smash Vet) More friends for Chibi! Good point.
Lucario (Smash Vet) I feel like there's a better chance for Mewtwo but Lucario would be nice Both he and Mewtwo would be amazing to duke it out in SSF2.
Lucas (Smash Vet/Highly Requested) I could see him My Brawl main.
Villager (Smash Vet/Smashville) YES! There was also a leak with him and Young Link last year.
Rayman (Why not?) A bit iffy on him but possible. 50/50.
Duck Hunt Dog (Another retro character/Smash Vet) We could use more retro characters. I love this game so much.
Ice Climbers (Smash Vets that need justice for SSB4) Meh. People want them in, and I am one of them. Plus they were my mains in Melee.
Monkey D. Luffy (I think we all know why everyone wants him)We're going to be good friends We're already pals!
Ganondorf (Smash Vet/We need more villains) I can see him with new moves. Agreed.
Mewtwo (Highly Requested/Smash Vet) Most Likely. Who doesn't want Mewtwo?
Diddy Kong (Smash Vet) Why not! Donkey Kong needs his sidekick.
Pit (Smash Vet) Who doesn't want him in? Brawl vet who deserves a spot in the game.
Zero (My main in SSF1) YES! Even though Mega Man X is not in the game, he can still get in.
Falco (Smash Vet) I hope he gets a few more changes. The Devs can get creative with his moveset.
King Dedede (Smash Vet/another villain) I agree with you a hundred percent. We're going to be best pals!
Ike (Smash Vet) YEAH! YEAH!!
Snake (Shadow Moses Island is in the game) Very, very, VERY likely It would do him justice for SSB4.
Olimar (Distant Planet is in the game) I feel like someone else would have a better chance. I agree, but he is very popular, plus Pikmin is one of my favorite games of all time.
Cloud Strife (With his inclusion in SSF1 & SSB4, he should be a guarantee) I agree. YEAH!!
Chrono Trigger (I hope he makes it back because he is one of the best characters ever) Yes! YES!!
Knuckles (SSF1) I'm leaning a bit more on him than shadow (Though I prefer Shadow) I agree with you on this one.
Shadow (We need Shadow!) I like him better. I would love both him and Knuckles in the game.
Simon Belmont (Heavily requested/Dracula's Castle is in the game) I don't know about him. Simon Belmont is from Castlevania. http://castlevania.wikia.com/wiki/Simon_Belmont
Skull Kid (It is about time Zelda gets a newcomer) Meh. I agree with you, I would rather see another Zelda rep get in like Ghirahim or Impa.
Daisy (Even though there are many Mario characters in the game, and two on the way for Beta, it would make sense to put Daisy in the game since we all love her (Sarcasm)) Recolor! This is the only one I disagree with you on. Daisy is very capable of having her own moveset and her own animations.
[/b]Young Link (It would be nice for the Devs to create his Majora's Mask moveset) Meh. I agree with this one, but in case Skull Kid doesn't make the playable cut, I think it's safe to say he'd make the cut.
Lip (For an obscure character, Lip would definitely make the cut in my eyes) IDK who she is. http://nintendo.wikia.com/wiki/Lip
Dillon (This dude was an Assist Trophy in SSB4, so the Devs should upgrade him into a Playable Character for SSF2) What would his moveset be? http://pre06.deviantart.net/bcaa/th/pre/f/2013/151/0/0/dillon_by_nobunagasmash94-d5yh9pi.png (Somebody's moveset for him, the credit goes to them).
Geno (I wish this dude would make it in) True YEAH!! Although if he were to be an Assist Trophy, I'd be fine with that.
Little Mac (Smash Vet/HEAVILY REQUESTED) Awesome Character. The best character in the game.
King K. Rool (50/50) We're friends now. Lol.
[/b]Andy (Advance Wars would represent the Game Boy Advance days, so the Devs should add him) Interesting. http://advancewars.wikia.com/wiki/Andy
Bowser Jr. (This dude barely made it into SSB4, so maybe he can do it for SSF2) [b]I hope so. I agree with you.
[/b]Wolf (Smash Vet who needs justice for being cut in SSB4) My main in Brawl I kinda have a feeling that Falco will be the last rep from Star Fox and they save Wolf for DLC.


Overall
Nice