I sincerely doubt Sakurai will add a character from a series just because he looked at the roster and said "you know, this series NEEDS another character!" because...why?
New series first. Zelda is fine. DK is fine. F-Zero is fine.
I think a common misconception here is that the roster is based off popularity, rather than as a celebration of Nintendo's history. Diversity is more important, and it's why we're getting characters like Ice Climbers and R.O.B. It's why Wii Fit Trainer makes perfect sense. It's why Mii, Shulk and Chorus Kids will make perfect sense. Obviously the two aren't mutually exclusive, but since you want to cover as many bases as possible, it's better to go for new successful IPs and historical characters than add another say, Mario or Zelda character because they're popular. Smash is the kind of game that INTRODUCES you to a character you're unfamiliar of, so that you can look at them and say "hey, this guy is cool, I wanna play his game!". It's all promotions. Obviously having the main villain, supporting characters, etc for very popular series is important but we pretty much covered all of that already except for Ridely. (But then Metroid IS pretty niche as is. I'm iffy on this one.) The only series this doesn't apply to is Pokemon, since it's kind of its own thing.
Diversity is Smash's biggest strength. You want to expand on it. If the roster just has characters from popular franchises just because they're from these franchises, then it makes no sense. In this case we'd have like 10 Mario characters.
Smash 4 is doing a very, very good job at this, Rosalina aside who is the only one possibly added just because of popularity. (She's also a good choice and she applies as well because she represents the Galaxy games, one of the most highly acclaimed games of all time.) I may like it so much because I played all the series represented so far (Animal Crossing, Wii Fit, Punch Out and Mega Man), but it's this kind of thing that I play Smash for. Truthfully, I want to be introduced to characters from franchises I don't know about, because that's what got me into Smash. That's what got everyone into Smash. It's not celebrating a specific series, it's celebrating Nintendo. As a toy company.
Sorry if I'm not making myself clear, I'm tired and and need to sleep lol.