Sales keep getting brought up in debates because they play a role in whole gets choosen. Sales are the main reason Duck Hunt was made playable and why they decided to include the character from "the very beginning of the project", however if the Duck Hunt character had no potential for a good moveset they probably wouldn't have gone through with adding him. The idea of a trio is what made the concept of the character work for Sakurai. So if a franchise has great sales and a character that would have a workable and interesting moveset, that character probably would have a better chance than one from a franchise with not so great sales.
This is very important to consider.
Yes, of COURSE people's primary reasoning for characters is wanting to play as them. But to say sales DON'T play a factor is a bad argument. Does anyone HONESTLY believe Wii Fit Trainer would've gotten in the game if sales weren't a factor? Wii Fit Trainer doesn't even have that original of a moveset, its just a variety of yoga poses for pretty much everything.
Say two characters with an almost identical gameplay gimmick could be considered from two separate franchises. Which would be more likely to get into Smash? The more popular character from the more popular franchise. If the franchises aren't too far apart in popularity, they might consider the character that's more popular or more requested, otherwise it would come down to personal preference.
Obscure characters are all fine and dandy, but the odds of getting them are slim-to-none.
Also, Abraham Lincoln is a historical figure. I know people like the idea of him, but honestly, there's probably next-to-no chance of him being playable. Henry Fleming is the more viable candidate. (Plus, Lincoln doesn't fight outside of A.B.E. anyway, though A.B.E. could be a Final Smash.)
Anyway, I pick characters that I like. However, characters who are obscure, even if they are ones that I like and can provide a good gameplay mechanic, I don't bother bringing them up (or rarely do so) if there's almost no chance of them getting in. I'd gladly request my favorite game character, but a one-off Zelda character unlikely to appear in future games is really kind of pointless to bring up (though at least she's got a trophy in-game :D).
Otherwise, my most wanted characters are K. Rool, Isaac, Dixie Kong, and Lip. Even Lip is obscure, but she was the main character of the original Panel de Pon and is the most recognized character from that game, and her wand is already an item. So there is that. If I went by gameplay exclusively I'd replace Dixie with Ridley, only because there's no guarantee Sakurai sees Dixie as a fully original character since in the classics, she does largely play similar to Diddy, just a bit slower and with a ponytail whirl.
K. Rool uses all kinds of gizmos as gadgets. He'd be a great trap-based character, there's few of them as is, and K. Rool's got the Blunderbuss with tons of potential for that alone. Isaac also has Psynergy, especially earth Psynergy, as there's almost no earth-based attacks at all. A new nature-based character would be nice; Lip could do that, but there's a couple of GS attacks as well. Plus a proper puzzle game representative to attack with Blocks, that's a good gimmick and style for Lip to use.
But the other argument is that you combine both sales and potential. There's numerous potential for additional DK characters, and its a popular franchise. Of course sales are going to come into play. Pikmin, for example, is decently popular, but I find it difficult to figure out a good secondary character from that franchise.