Personally, I think if you already have a spot in-game, you have practically no chance. But that's just me.
If I'm Sakurai, and I'm looking at this ballot post-completion...and I see a Top 10 list of: K.Rool, Isaac, Wolf, Snake, Inklings, Ice Climbers, Ridley, Wonder Red, Dillon and Shovel Knight...I'm going to be looking at things that I haven't already considered or aren't already in the game.
K.Rool is a top choice, isn't available as anything but a trophy, etc...why should I pick a character who my developers have already coded into the game as something else? That's essentially time I've wasted developing the game.
Now, I approve of Takamaru, Dillon and others as Smash characters...but for future installments. People have to buy this stuff. Sure, you could argue that people will buy anything. But my issue still lies with the time wasting in it all. Sakurai and the dev team very likely had meetings to make these decisions. If he didn't think Waluigi is playable material during development, he will probably STILL believe that. A bunch of people voting for him to MAYBE crack the top 25 probably will not convince him otherwise. I think Sakurai is a great developer, but he can also be stubborn. I don't blame him sometimes.
At the end of the day, the Top 10 characters are probably going to have the strongest shot of getting into the game, if that's the goal of the ballot. So to me, that includes K.Rool, Isaac, Snake, Wolf, Shovel Knight, Inklings, Ice Climbers (probably won't happen because tech issues), and a few others. Maybe think into the next 5 to make it the top 15 choices.
Expecting an NPC to become DLC isn't impossible, but it isn't practical. You could literally say, "ANYTHING CAN HAPPEN!" but it's still a real bad argument. It's like going into a murder trial and saying, "It COULD have been someone else. ANYTHING COULD HAVE HAPPENED!". It's not really something that people take seriously.
Smash 4 DLC is still Smash 4. Using Little Mac as an example doesn't suddenly invalidate the idea that NPC don't have much of a chance for DLC. That was between games. So several years between installments...not months to a year and a half since release. It's definitely not the same thing.