Personal thoughts: three more slots is not the end, merely the next wave.
I think we have a few types of characters.
There are characters I just assume have been worked on: Ice Climbers and Wolf at the top of this list. Not that they are necessarily in the game, just two options that I think are possible. Neither can actually win the ballot choice (to be fair, no veteran can. Not without the entirety of the ballot being put into question). Ice Climbers, if in the game, would also be a poor choice to be paid for (we know they were set for base game, and the goal of DLC is explicitly stated to be NW content, or at least to appear that way). Appearances and PR matter-- particularly for something so well publicized for Nintendo.
Then we have other characters who are, well, grouped together, at least in my mind. Banjo Kazooie, Dixie, and King K Rool, being a set of these. I could see two of them getting in, sure- but being born of the same family, they are not different enough to be the sole choices. Again, just my perception, but regardless of any facts we all spout, we go mostly by impulses and desires for this game.
Final category; others. This would be for third party and shock characters. I know some renound leak finders may believe all the dlc is for third party- but, still, I think this would leave a poor taste in regard to what the ballot is. No Nintendo Star Power would, I believe send a message Nintendo would not condone, in this era where it is capitalizing immensely on their IPs (cross overs galore, mobile games, possible movies and TV, theme park...)
All things considered, I think we will get 6 more characters. Two more waves. My hope is that they will be in October and in December. It is, of course, hopeful- but seems to follow suit with DLC campaigns so far.
Randomly now>
Round one: Chibi Robo, Professor Layton, Wolf
Round two: Inkling, Captain Toad, Dixie