The only two real problems I see with K. Rool or the Inklings getting in this time around are as follows:
1. Both characters require a commitment to make. They're both pretty complex to build.
2. Both characters would be a huge deal for Smash 5/6 as newcomers, especially the Inklings since Splatoon is Nintendo's next big franchise (until Yo-Kai Watch takes off in the Western World, which even then isn't owned by Nintendo).
As a businessperson, I would honestly hold off Inklings for Smash 5 or 6 at this point. Given the thin roster of potential newcomers For Smash 5 we have, the Inklings would be to Smash 5 what Wario, Diddy or King Dedede were to Brawl in a few years' time. Inklings would be a lock for a newcomer, yeah, but the anticipation and slow burn makes hype, which equals cash. Plus they'd be more established, have more air breathed on the franchise, and likely have a sequel to Splatoon forthcoming.
K. Rool is a different beast. Making K. Rool DLC would put some focus back on the DKC series. On top of that, it would garner hype for Dixie Kong for Smash 5. Without a fellow DKC series "rival" to "compete with," Dixie hype would run rampant and without boundaries similarly to say Lucario hype in Brawl (although probably with less attention, because Pokémon hype is ginormous and unconquerable).
Almost all the focus from said DKC fans would be on Dixie. This would make Dixie into a bigger and more notable draw; possibly bigger than what K. Rool was as pre-release Smash 4 prospect, since the entire DKC fanbase would be focused and united. Given that future DKC titles are gonna happen here within the next few years and Dixie is gonna be in them, that only adds to this.
That is my analysis at least. For the record I am indifferent (chaotic neutral of sorts) towards Inklings (and Shovel Knight for that matter; worth mentioning because he's in lots of leaks lately) because I haven't played their games (enough), while obviously I am a K. Rool supporter. Still, I like to think my logic is without any real traceable bias.