I just had a good idea for a potential DLC world. What if the level takes place on a vacationing island/resort and a lot of the characters from the main game are there with Yooka and Laylee celebrating the defeat of the main villain? Sort of like the Treasure Trove Cove-like island at the end of Banjo Kazooie, but much bigger. Maybe some of the characters can give you cool extras after you've obtained a certain amount of Pagies, and maybe Rextro Sixtyfourus will unveil his latest game to you after you've obtained everything you can collect (just like in N&B when Klungo let you play Hero Klungo Saves Teh Universe). It also gives you a chance to see what the main characters have been up to since the end of the game. Maybe after the financial success of Yooka-Laylee, Trouser was able to buy himslef a fancier pair of pants. And maybe Kartos was able to get that oiling and tuneup he'd been needing for a while and offers to give you a much faster (*ahem*, harder) ride around the map.
The theme of the entire map would be a reunion and celebration between the characters. It doesn't matter that this world is only accessible after you've beaten the game, because this is the kind of world that can only be enjoyed afterwards. I know it's a very rudimentary idea, but I feel like Playtonic can expand it and make it amazing. Whatever they plan to do, I'm sure it'll be amazing.