Hm, back as far as Melee I was always confused why only Donkey Kong was in the game. Where was Diddy? Or Dixie, Lanky, Tiny, Chunky for that matter? I hadn't even considered K. Rool at that point as villains in Smash were still a very rare thing back then.
When Brawl came around, we got Diddy, and I think that was enough for most people. He might have even been overdue as far as some were concerned. Dixie was part of the forbidden seven, but we don't know if it was as an original character, clone/semi-clone, or just remnants of when she and Diddy had been a duo character. K. Rool was only barely on the radar, as a dream pick but not one anyone remotely expected.
Then came Smash 4, where it felt like the roster had expanded to the point where 2 reps felt like too little for Nintendo's 4th best-selling franchise after Mario, Pokemon, and Zelda. 5th if you count Wii Sports/Fit/etc. Then it was a battle - the unlikely but heavily requested K. Rool, or the seemingly likely but somewhat less popular Dixie? Turns out we got neither, even with 20+ newcomers in that game including DLC. It's a shame, but you can't please everyone; Metroid got the same treatment, and Star Fox actually lost a rep.
And now here we are in Ultimate. The sentiment that it's by and large a "fanservice game" is very much agreed upon, after "Everyone is Here!" and the Ridley reveal. As we've all said before, this is a great sign for any character who's ever been a huge fan request, from Geno to Banjo to Isaac to K. Rool. The only problem is adding all the veterans took so much work that Sakurai even felt the need to say not to expect too many newcomers this time around. This is serving to breed more "competition" as the "slots" for potential newcomers are smaller than ever.
With all that being said though, I think, after a decade of waiting, this will be the time for DK fans. Whether that means they listened to the demand for K. Rool, they play it "safe" with Dixie, give us both (Dixie echo/semiclone, or maybe one as DLC), or they throw us a completely left-field curveball like Lanky we'll at the very least be getting another Donkey Kong character in this one.