Part of me wishes they would focus all of their attention on their next game so we can get it as soon as possible, but another part of me likes the idea of having something to look forward to short term. I like the idea of alternate visuals, but I don't think they should be limited to different costumes for the main characters. It'd be awesome if they had certain themes (like Halloween or Christmas) that changed how the entire game looked. I also like the idea of bonus multiplayer maps and modes.
I'm a little undecided on worlds, though. Like some people have said, it doesn't feel right to have worlds separated from the main story. Maybe I'm just old fashioned and want things to be done exactly how they were in Banjo Kazooie, but I feel like the worlds would lose a lot of character if they aren't placed in the main story somewhere. I feel like every world has its own place in the game's story and progression. After experiencing the happy levels of Mumbo's Mountain and Treasure Trove Cove, most people would be surprised to see the dark and gritty level of Clanker's Cavern, giving the level a "this is new" feel, and makes you wonder what else the game has in store for you.
I think Creepy Castle, the final level of DK64, is by far the best level in that game. The level is dark, scary, and gloomy, and it gives you the feeling that something big is going to be happening soon. If the level turned out to be a bonus you got after beating the game, I feel like it would have lost a lot of its seriousness, since you know nothing big is going to be happening. The level would just be dark, scary, and gloomy - there'd be no story, progression, or tension to keep you on the edge of your seat.
Click Clock Woods is another good example. It's pretty much the polar opposite of Creepy Castle. Before it, you have worlds like Gobi's Valley, Mad Monster Mansion, and Rusty Bucket Bay, which were all pretty serious levels. So, going by this pattern, you'd assume that this world would be pretty serious, too, right? Wrong! The game does a complete 180 and creates the most happy and jovial world you can imagine. The game tricks you into thinking it's becoming more serious in preparation of the epic conclusion, but throws a curveball that reminds you Banjo and Kazooie's adventure is a lighthearted one. If this level turned out to be a bonus you obtained after completing the game, it would have lost that magical "happy end game" feel.
I just don't think any bonus levels in Yooka Laylee will be able to capture any of the feelings I will have had with the game's main levels. There'd be no "I wonder what is next", "change of pace", or "big conclusion" feelings with them. Of course, I'd be happy playing any sort of bonus content we get with this game, but I can't help but feel they're better off putting their resources towards making a new game.