TL;DR post. How do you do spoilers?
IMHO, the only ones at the moment that can be considered true possibilities are Lucas and Wolf, and that's mostly because they could work on the Brawl models to work for Smash 4, where Sakurai's "It's hard to port stuff from Melee." sentence doesn't have any effect(or whatever that word I'm looking for is) and if you think about Sakurai commenting that he does not like to cut characters. With Lucas and Wolf, they are Nintendo-property unlike Snake and most likely won't have issues with the 3DS unlike Ice Climbers. Otherwise, when you look at characters who haven't had a past Smash appearance, it's hard to know which ones are actually possible and which ones are not.
Like despite people saying that Isaac may have a chance due to Golden Sun not having any trophies or Isaac not being an Assist Trophy anymore, you can't exactly call him an actual possibility. For all we know, Nintendo isn't even thinking about him once or don't think he is unique enough. We may say Isaac has a 50% chance, but to Nintendo, he could be a 0% chance for all we know.
With Lucas and Wolf, since they were past characters, Nintendo had to think about them at one point during development(especially if one looks back at Gematsu and what it said about Ness & Lucas) and with DLC, Nintendo would most likely look back on them again to see if they should be DLC or not. Heck, with Star Fox Wii U in development and looking back at E3's little Robot Chicken skit mentioning Mother 3, it wouldn't be surprising if Lucas and Wolf were used for promoting a possible Mother 3 VC release and Star Fox Wii U, which a lot of people have been thinking as a possibility for the Inklings and Splatoon.
But I guess that's the beauty of waiting to see what they may do for DLC. After all, even if characters like Dixie and Isaac aren't guarantees, it feels so exciting if they possibly do get announced. Heck, the fact that it works for even for the more obscure characters makes it even more exciting, whether it'd be a classic character or modern character.
Or at least that's my opinion. Maybe somebody else thinks differently. But regardless to me, the kind of person who really doesn't care about which characters get in and is excited for any announced character, whether they be a classic, an unknown modern or a familiar face, only the thought of another DLC character happening is where my hype rises and all I hope is that Mewtwo is not the only one.
That being said, I must be the only one who hopes for DLC costumes and character costumes. I think I was spoiled by the Koopaling and Alph costumes, despite them not being separate characters. Like if Lucas becomes a character again, Kumatora would be an interesting costume for Lucas. And I'm also hoping for possibly a Felix or Matthew costume for Isaac.
Also, looking at the fact they almost made Lucina a Marth costume, I think Priam(Paris) and Morgan(Mark) would be obvious DLC character costumes for Ike and Robin. Heck, Fire Emblem could really benefit from character costumes, due to a good number of characters having similar builds and fighting styles to the current characters. Marth is pretty much the model for Leif, Eliwood(before promoting) and Eirika(before promoting). Lucina could work for Seliph(and ironic as well). Robin could work for Katarina. And if you found a way to "replace" Ike's fire effects, he could work for any Hero character who uses a big sword.
As for Roy and Lyn, while I know alot are asking for them, I feel their best bet is if Intelligent Systems does a remake for FE6+7 around Smash 5's time. Considering how successful Awakening was, that FE7 already has the Tactician and that they could dp the children thing through FE7->FE6, I wouldn't be surprised if they did a FE6-FE7 remake at some point. Heck, they wouldn't need to worry about removing Robin's moveset and use that for the new Tactician and even keep Robin as an Alt for them. An FE6-7 remake sounds rather likely, but when it will happen is another story. But if it were to happen shortly before the new Smash is announced, that would likely be Roy and Lyn's best chance at becoming characters for Smash again.