WayForward has been very adamant when it comes to paid DLC for Shantae and the Seven Sirens. The team has said that they won't be working on paid DLC, but there's actually some free content in the works! WayForward's Matt Bozon teased that content in an interview with Nintendo Life.
We decided not to do paid DLC, or an “Ultimate Edition,” for this game. That said - content updates are free and are available to everyone! We don’t have any specific announcements today, but I can say that we have ideas for some fun little free add-ons in the works! We’ll have more info on that in the coming months.