I find it very strange though that the only appearance K. Rool and Dixie Kong had since Brawl was in one Mario sports spinoff. By comparison, between Melee and Brawl there was a slew of games that they appeared in. However, nearly all the games that K. Rool appeared in were by Paon (King of Swing, Jungle Climber, Bongo Blast) and all the games Dixie Kong appeared in were by Paon, Donkey Konga games, or Mario Sports titles (Baseball GCN, Hoops 3-on-3) as well as Diddy Kong Racing DS.
That they just disappeared off the face of the earth after Super Sluggers is very strange. The only other Rare characters that were used for Returns besides Diddy were Cranky, Rambi, and Squawks. Moreover, Dixie Kong was one of the few characters that was playable in Mario Hoops 3-on-3 but did not return for Mario Sports Mix.
The only chance I see for K. Rool and Dixie Kong to make another game appearance before Smash 4's release would be if Retro manages to get out another DK platformer before then or if Paon is making another (mediocre) DK title for the 3DS. We have another two years until Smash 4 releases (assuming Smash 4 is released Q4 2014), so its definitely feasible for K. Rool and Dixie Kong to appear in another game before Smash 4 releases. Though for the time of Brawl's release to the time of when Sakurai is done figuring out the roster, K. Rool and Dixie Kong have been absent.
I have also been thinking about Mario newcomers. None of the candidates seem like they could be made into particularly interesting fighters (Toad, Bowser Jr., Waluigi, Dr. Mario), and while they are all very recognizable and more well-known than practically all the possible newcomers for Smash 4, they would not be able to stand out much on the roster.
Then you have the fact that Star Fox and F-Zero went into hiatus years before Brawl's release, which would make any newcomers for those series problematic.
I am starting to think Waddle Dee (Bandanna) is looking more and more like an attractive choice for Sakurai's team. He is one of the few characters from a veteran Nintendo franchise that has:
1. Had a prominent role in a game released between Brawl and the beginning of Smash 4's development.
2. Not yet playable in Smash Bros.
3. Already has an aspect to him that would make him easily discernible from the rest of the roster (spear-user).
The spear-moveset from Return to Dream Land already has aspects to it that make it look like it came right out of Smash Bros. The biggest problem though for Waddle Dee though is:
1. Lukewarm demand for the character.
2. Sakurai not wanting to include another Kirby character out of humbleness and/or feeling the Kirby series is already complete now with the addition of Meta Knight and King Dedede.
However, despite these two, Waddle Dee looks better than most characters from veteran Nintendo franchises with the exception of characters from the Mario series, Ridley, and Palutena. That is not much, especially when you exclude Mario characters (most of whom are uninteresting). So Waddle Dee is basically up there with Ridley and Palutena in terms of having a major role in a game released since Brawl.
Think about that. Since Brawl, the only unplayable major veteran series characters that have had important roles in their series since Brawl's release (outside of those from the Mario universe and are not flavors of the month like FE/Pokemon) have been Ridley, Palutena, and Waddle Dee. And we wonder why Sakurai is deciding NOW (and not for Smash 5) that Smash Bros. needs a massive overhaul.
From B-tier series, the only two that are looking pretty are Little Mac and Shulk. Sin & Punishment 2 and Golden Sun 3 were big time flops.
Really, I think the pool for newcomers for Smash 4 looks a lot worse than many people think.