I think they're going to want to stick to big names and able sellers as DLC. Which means bigger names. When the Chorus Men were on the 2nd Gematsu list, hardly anyone even knew who they were. And this is the hardcore Smash community, the one that knew all about super niche Xenoblade. Rhythm Heaven may be big in Japan, but there are two other regions that have little idea about what Rhythm Heaven is. And because DLC is sold piecemeal they cannot release a character that only 1/3rd of their market has heard of, but that doesn't matter if they're on disc characters as you buy the full game, not part of it.
Lucas, Mewtwo and Roy were already Smash characters, therefore already immortalised. Neither Roy or Lucas would have ever been big enough to be DLC characters without having previously been in Smash (remember that Smash within itself is one of Nintendo's biggest franchises). Mewtwo was pretty much the most requested character the world over and Ryu is freaking Ryu. There's not a gamer alive who doesn't know who Ryu is.
Mario Kart DLC is also a prime example of this. They released Mario, Peach and Bowser as the Mario DLC characters over characters like Birdo, Dry Bones or Boo.
This is why I think well known characters are the most likely newcomers (and possibly why K. Rool has probably been passed up). Inklings, Rayman, Paper Mario, Waddle Dee and Dixie Kong are probably the 5 most safe selling newcomers they could pull out right now. As well as a few other key 3rd Party's.