Since we are on stages. Spiral Mountain or Top of Grunty's lair? I like the latter just to have a Grunty Boss battle.
Pretty much what EpicElliot and Lycan Rex said, I'd personally want Spiral Mountain, as it appeared in Banjo-Kazooie, updated in HD rendering, of course, as a moving stage like Delfino Plaza, the Halberd, and Wuhu Island (among other traveling stages) where you fight in each major location of Spiral Mountain, starting off in front of Banjo & Kazooie's blue house, randomly going to different parts of the hub including the vegetable garden where you learn how to Rat-a-Tat-Tap and Claw Swipe and a carrot, onion, or cauliflower randomly pop out (as well as appearing throughout the stage randomly as well), the stumps where you learn how to Flip Flap, the quarry where you smash rocks with the Beak Barge, the little bridge in front of the waterfall, the platforms leading up to the waterfall, atop Spiral Mountain in front of Gruntilda's Lair, and then finally atop Gruntilda's Lair where Gruntilda's waiting to taunt you with insults, maybe give you a trivia question or two, and attack you with magic! Was also thinking Jinjos might pop up at random that if one player manages to successfully collect them all before anyone else, the Jinjonator is summoned to dispense Gruntilda and any players who were unable to rescue all the Jinjos! Might be a bit much, but just goes to show there's a lot of potential for Banjo & Kazooie and their stage, which doesn't even necessarily have to be Spiral Mountain, a Gruntilda's Lair or Isle of Hags stage would be cool too. But for my money, I personally think Spiral Mountain, while not the most extravagant option, is the best representation for the bear and bird's home stage in Smash. They could even have characters from Banjo-Kazooie, Tooie, and Nuts n' Bolts show up to root on the fighters throughout the stage, like Bottles, Tooty, Mumbo Jumbo, Brentilda, the Jinjos, Boggy, Gobi, Humba Wumba, Jamjars, King Jingaling, Mr. Fit, and even Trophy Thomas!