I wouldn't mind it, but I think the reason why it doesn't happen is because even in spinoffs Bowser is the only "main antagonist" represented, and its intended that way on purpose. King K. Rool, however, still represents the "main antagonist" in all DK spinoffs that happen (except for Jungle Beat), and was planned to featured in additional cancelled games like Diddy Kong Pilot. So its definitely not out of the question for K. Rool to appear in those kinds of games, but Nintendo doesn't like him appearing alongside Bowser it seems. And recently, DK has been the only Donkey Kong character to appear in Mario spinoffs anyway.
They've repeatedly denied Diddy Kong appearing in Mario Party, he's been cut from Mario Kart, and although he is in Mario Golf World Tour its the only Mario sports spinoff Diddy has been in recently. I think they're going to try and bring back DK spinoffs and separate the franchises more, so only DK himself appears in Mario spinoffs, while all the DK characters (who numerous people keep saying have no connections to Mario at all except DK, which is true), will instead be kept exclusively in DK games and DK spinoffs, and I'd bet that there's a high chance of DK spinoffs to come back again.