I suggest this not because I think its existence would substantially better the game we are already receiving, but because something along the lines of the concept is rather inevitable once the game releases to the general public.
Playtonic have been very firm in saying they have no intention to override, or 'replace' the lineage of Banjo games so to speak; as they've said, "Banjo is a Microsoft property, Yooka-Laylee is a Playtonic property. There is no real 'connection'". The parallels are blatant, and the intention to make the Banjo Tooie sequel that never was is rooted in every part of the game, but it is a 'spiritual successor' nonetheless. Despite this, depending on how supportive the final release is of mods, there will no doubt be some very talented induviduals on the scene to seamlessly bring Banjo and Kazooie into the game, along with all the Banjo assets corresponding to those present in Yooka.
I also think by now it's safe to say that Microsoft have little reason to make a proper Banjo, now and in the coming years. What market there was to make a tried and true successor to the series has already been masterfully cornered by Yooka Laylee. The profits Microsoft accept from its release on Xbox One are enough of a justification to not try and steer Rare away from their current plan to produce a line of ambitious new IP, no matter how successful Yooka goes on to be. Additionally, what can be said for sure is that we will never see the Banjo Threeie that the Rareware of yesteryear would have made. Undoubtedly the closest we will ever get to that game is thankfully releasing early next year.
With all that being said, would it be a worthwhile venture to round up all the willing talent under some sort of centralized effort to make a mod that entirely turns Yooka Laylee into the Threeie we all expected? I'm talking inside and out, as if it was 2005 and you just picked up Rare's Banjo Threeie for the Xbox. Of course you'd have to rework the plot to be a direct continuation of where Tooie left off, swap the main characters (BK for YL, Mumbo for Puzz, Jamjars/Bottles for Trowzer, etc), maybe integrate Spiral Mountain at the beginning somehow. The bulk of the game, and the individual levels themselves would remain very much the same, however. Really it's only the story and characters you would need to alter (swapping Capital B for Grunty, if that were the plan, would be quite the challenge, but in thinking about it I had a few ideas of my own). From what we've seen so far, the move sets of YL and BK are certainly not a 1:1 match, but it's still more than enough to work with. And of course, nothing could really be efficiently planned out until we've seen the entire game, once it releases.
Like I've said, it would do little to enhance the game itself, essentially it would just be a nice bit of fan-made-fan-service. I also don't know if, should it go far enough, the project would run into legal trouble with Microsoft. I can't really see that happening, because there does not exist any Banjo games for the mod to be a competitor of, a la Project M with Smash U. There's no scenario where Microsoft would be losing any money over its existence. If someday that became the case, and we did get a new Banjo Platformer, well then this would be an entirely different story.
I'd just like to gauge all of your feelings on this idea, or if you think such a project could even come together. It's just one of many thoughts I've had brought about by my eager anticipation of this game