Well let me put my basic methodology (or something that loosely approximates a methodology) on the table: I think that the two main things to keep in mind when discussing possible DLC characters are 1. Will this sell and, if so, will this sell more than other characters that we could make and 2. How easy is it to make this character; that is, when you see two different characters that are likely to sell equally well the one that requires less effort is more likely to be chosen. I don't want to seem overly cynical, and I readily admit that there are other factors that are obviously very important (whatever Sakurai thinks will be fun to do, mostly) but I think these are good general guidelines to more closely approximate what's going to appear.
Hard to argue with thinking that the ones who would sell highest would be the DLC choices- I just don't think we agree on what would sell highest. The core smash community would buy any character, regardless. Catering to them is actually counter-productive as far as DLC goes. When making DLC you want to reach the most broad audience as possible, and in a game like Smash Bros I fully believe that is with what is fresh and new (something Sakurai has been shown to like in general, never just recreating).
As far as veterans go, the single piece of evidence we have for why they may return is that Sakurai has said he doesn't like cutting characters because they have fans. That said, we know he already cut them, and it is clear cuts must be made.
Every other bit of evidence points toward it being new content, fully. Two years ago Miyamoto made a statement on what Nintendo's quest into DLC would be like:
http://kotaku.com/nintendos-strange-two-year-journey-into-dlc-1641525034
He explains here how he wants Nintendo games to have DLC that feels new, fresh, and
not like with held content, which veterans are the epitome of. Sakurai echoed these views a few years later when outlining what his concerns were about DLC in Smash. Nothing should be strange about this, though, as Nintendo is constantly on the verge of trying to Innovate.
Even still, we have had other issues. Sakurai has gone on record to say that characters get lower priority when they are not likely to reappear. Roy and Pichu certainly fit here (they appear, maybe, but in no notable way). Lucas's creator went on record to say that his series is over.
That said, I think that these more casual fans are more likely to be fans of Smash Brothers specifically than say, Fire Emblem or anything but the most prominent Nintendo franchises. It's anecdotal evidence but I remember a ton of people in college who only bothered with Smash Bros and a couple of other games on their Wiis, and many that picked mains that they wouldn't have cared about otherwise. These are the people that would instantly snap up a veteran that they recognize over some -perhaps more 'deserving'- newcomer they've never heard of.
That's exactly my point though (well, one point)- there are characters Nintendo has access to who are more recognizable than ones who were in previous editions of Smash. Even if Lucas was in Brawl, even if Roy was in Melee, Dixie Kong and Captain Toad are more well known. If we go with purely well known, the DLC will be strictly Pokemon and Mario.
The veterans who are unquestionably the most recognizable to the highest number of people are Snake, Squirtle, to a lesser extent Ivysaur (who is then just a misappropriated Bulbasaur). Those aren't the ones the forums ask most for.
My second guideline more clearly points to veterans - they seem to be able to import data from Brawl that will make development much easier (something very important to note considering a project lead by someone so clearly burnt out from overwork as Sakurai!) and those that haven't been in since Melee (Pichu, Roy, Young Link) can easily be made as clones.
Sakurai has stated in multiple interviews that data was not ported, though. He made it clear each character was worked from the ground up, with the exception of Dr Mario, Lucina, and Dark Pit- mostly because of the balancing being easier off of them. You may now have a point for why the missing Melee veterans have a shot as clone entrants, but certainly not for any other veteran.
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