Spin-offs would be nice, but with how scrawny the recent Mario spin-offs feel now, I can't imagine how anemic a DK spin-off would be.
Given how solid the GCN and Wii Mario spin-offs are, is it possible that the development cost surge of HD's advent threw a wrench into the usual development of Mario spin-offs?
It pretty much threw a wrench in the spin-off quality and quantity of many franchises.
The shift to HD development ended up being a struggle for a lot of companies, particularly Japanese ones. The 7th and early 8th gens are filled with fairly competent developers going through a bit of a low point as the cost of HD titles (and in some Eastern cases ill advised attempt to Westernize) let to a few disappointing outings. Simply getting titles out on time and under budget was much more of a challenge and in some ways the constant "real is brown" phenomenon was the result of companies going for the popular safe aesthetic because flops were now much more devastating due to the involved resources. Even the Wii-U's eventual failure was arguably somewhat rooted in Nintendo not having the same ability and speed to get out major games with any real frequency to the point where the market simply didn't see a real future for the system.
The eventual loss of non-mobile portable games hasn't helped. A lot of creative experiments were possible because general development on the GBA/DS/3DS/PSP/Vita was generally cheaper and (especially for Nintendo) often had a bigger install base than home consoles. Its when you could see interesting side games from Mario, Sonic, Mega Man, Bomberman, Zelda, Final Fantasy, Fire Emblem, Metal Gear, etc. They were means of utilizing the brand name in interesting ways without necessarily being too damaging if the game failed. Its why I lamented DKC not getting an original DS platformer because it was the perfect console to take a small risk on with a new game. If it had worked out, who knows how many new fans it could have created?
Both of these factors help explain why AA games are only just now starting to come back with any real consistency (and as noted, the Mario sports ones are hardly the most complete releases at launch). There's no portable only option where a major franchise can put out modestly budgeted games for decent returns, so mid-level titles (now after 15 or so years of learning the ins and outs of HD development costs) are the most practical means for that unless you're just willing to do mobile stuff.