If it's not a rule, then why does absolutely everyone expect a DK character to happen just because?
With Inklings teased. And the current line-up of Nintendo first party All-Stars, the biggest names are already covered. DK has 2, Dixie Kong and King K. Rool, and Metroid had Ridley, which finally happened. Kirby got Bandana Waddle Dee left, and Mario has everyone ...
All-Stars comes from their impact, fan requests/love/service and how often they appear because they gain "popularity".
People were shocked Dixie did not make it in Smash 4 (me included), and there were a lot of krocodile tears fallen as well.
It's more of a shock that DK got nothing. No Assist Trophy, no playable appearence. Just a stage, which is weird as between Brawl and Smash 4, there was 2 Donkey Kong games which sold very well. For sure a DLC would be at it's place, but no, nothing ...
Now with Ridley, and Stage Toggles, and ALL veteran fighters come back, it makes sense to think that this time, Sakurai listens more to fan-requests, and DK newcomers have been on peoples mind as they freaked out, went bananas or gator crazy in the last game. Even non-DK fans feels it is time for DK and Metroid, and Metroid got their love, DK was naturally left.