OK, let's look at the timeline:
1) DLC was teased last November, with our first glimpse at Mewtwo's model.
2) Fast-forward five months to April 2015: Mewtwo is released, Lucas is shown, and there is evidence of Roy and Ryu in the patch. There are also two extra slots for characters.
3) Fast-forward two months to June 2015: Lucas, Roy and Ryu are released. No evidence of future DLC characters in this update.
4) Fast-forward two months to August 2015: What will happen? Tournament Mode will be released, and likely Peach's Castle 64 and Hyrule Castle 64. But what else might happen?
5) Fast-forward another two months, to October 2015: The Fighter Ballot formally comes to a close. What then?
It seems that every two months, there will be a new DLC development for Smash. Whether that involves characters, from here on out, is anyone's guess.
I'm inclined to think there will be more characters than Masked Man guessed in his blog post. Nintendo is so confident in the profitability of the DLC that they're running commercials for it in Japan. Sakurai has also managed to incorporate four characters so far — Mewtwo, Lucas, Roy and Ryu — along with new stages and Miiverse outfits, the YouTube feature, balance patches, etc.
So I'm a bit more optimistic — but how optimistic? Well, Sakurai has established the Fighter Ballot concludes in October. He may start work on a ballot-based character before then, but I suspect any character based on the ballot would take at least a few months to appear. He doesn't want to release a ballot-based fighter right after the polls close — if he does that, then everyone knows that Sakurai wasn't paying attention to votes near the end of the campaign.
And maybe they -will- already be working on the character before the campaign is over. Maybe the campaign is "just for show." Either way, they will want to maintain the appearance that 1) the ballot-based fighter was chosen after voting had ended, in October, and 2) that development on this character took place after voting ended.
That right there locks Sakurai into a commitment of at least a few months beyond October. In which case, we're looking at Sakurai working on DLC for July... August... September... October... and likely November and December, as well.
About five to six months... About the same time from Mewtwo's reveal to Mewtwo's release. A time during which they were also working on Lucas, Roy and Ryu. So in that amount of time, if they have Sakurai locked down this way, I think they would expect more characters. Sakurai knows the players would expect it, and unlike the stages, Sakurai has said nothing to lower our expectations. He didn't say, "I don't think we can add many more characters." I'm sure he's said it in interviews, but he didn't say it in the Direct. Instead there's a ballot that's open and ongoing, and the polls don't close until October.
Any (or all) of the remaining Brawl veterans seem possible. Easy money for Nintendo, if Sakurai is willing to put in the work and rebalance the game for them. They'll sell, and Sakurai can feel happy knowing the roster is even closer to the ultimate lineup, reuniting more fans with their mains.
I don't see why the CSS couldn't be redesigned, and simply activated for those with all of the DLC. It's hard to imagine how they could fit in all the characters, but I'm sure they could find a way to make it work. Perhaps it would be a matter of shortening the boxes for 1P, 2P, 3P, etc, allowing for another row. I don't know, but so far everyone seems to be thinking of what could fit based on the current design. It's possible they could just overhaul the layout altogether. And add more slots for more characters — I think they're going to be much more controlling with what leftover data can be gleaned from each update.
