I'd be cool with an edited opening sequence or file save select thing where a birthday (cup)cake is present. Maybe having Yooka/Laylee interact with it, with a little dialogue like:
Yooka: Hey Laylee, did you bake this?
Laylee: No, who's is that?
Yooka: I dunno. It's not your birthday is it?
Laylee: No, and it's not yours either.
Yooka: Well, Happy Birthday whoever you are!
Laylee: Lemme have a bite!
(Yooka brings the cake to Laylee)
(Laylee eats the entire thing in one bite)
Laylee: That was good!
Woaaaah, it finally came out!