I got this idea when I watched this video: https://www.youtube.com/watch?v=rJCh5j7slhs Originally Pikmin, the Gamecube game, had a single stage where you played as 2 characters instead of one. How about we do the same in Yooka-Laylee? One character plays as Yooka and Laylee while the other played as... err... BANJO AND KAZOOIE, because that totally won't get Playtonic sued by Rare! Actually, this is Generation 8, why can't we have 3 players! The third character would play as... well... DONKEY AND DIDDY KONG, because that totally won't get Playtonic sued by Nintendo! Realistically, though, Playtonic should probably think of two more original duos if they do this. How about:
Glocken-Spiel=Yak and Eagle.
Saxo-Phone=Crocodile and Seagull.
Vio-Lin=Deer and Pheasant.
Xylo-Phone=Leopard and Finch.
Casta-Nets=Dog and Pigeon.
Marim-Ba=Lion and Vulture.
Trian-Gle=Cow and Chicken.
Agi-Da=Monkey and Parrot.
Apin-Ti=Horse and Toucan.
Ashi-Ko=Cheetah and Buzzard.
Babou-La=Goat and Chicken.
Bongo-Drums=Gorilla and Duck.
The premise of the world could be the kingdom the other two duos are from is threatened by the villain of the game so Yooka and Laylee team up with them to help. If the villains a witch or warlock, maybe they've cursed the land or King or Princess or President or Prized cow.
How about at the start of the level each character has to choose a transformation that gives them a different boost, to encourage teamwork. You could do what they did in Jolly Roger Lagoon and add a currency to the level, which you have to use to buy transformations. Here are possible transformations:
Crown: Increase the chance of a defeated enemy dropping a honeycomb piece.
Bomb: Like the detonator in Banjo-Tooie.
Bow and Arrow: Shoot arrows at opponents.
Sword: Gives you a powerful slash attack.
Fish: Improves swimming.
Arcade Machine: Gives a top down view and shoots a stream of bullets like in an arcade shoot 'em up.
Honeycomb Piece: Increases your maximum health.
Rock, Allows you to swim in lava and walk through fire.
Boomerang: Acts like the boomerang power-up in Super Mario 3D Land
Snow Shoe: Stops you slipping on ice.
Four-Leaf Clover: Whenever attacked by an enemy, there's a 1 in 5 chance they'll miss.
Balloon: Stops you from falling.
Serpent: Allows you to turn to stone to protect you from enemies.
Mummy: Allows you to walk on quicksand.
Fan: Allows you to blow away enemies with gusts of wind.
Light Bulb: Lights up dark areas.
Ninja: Gives you a dash attack.
Flamethrower: Shoot fireballs.
Spinning Top: Gives you a spin attack.
As this world has a completely different gameplay style, it should be a bit larger, and more varied. It could have a woodland zone, a fire zone and a snow zone. There could be an NPC you gave you Pagies for completing special challenges in these zones.
To encourage cooperation all three characters would share one honeycomb meter. This stage would be available through local, download and online multiplayer. Of course, you can also play this world single player but it will be a LOT harder. This world also contains a versus arena where players battle for supremacy.
More character possibilities:
Agung-A-Tamlang=Orangutan, parakeet and beetle.
Ca-Jón=Cobra and fly.
Clap-Sticks=Chimp and dragonfly.
Hand-Pan=Worm and bee.
Steel-Pan=Rhino and sparrow.
Wood-Block=Stag and pigeon.