For as great as Shovel Knight is, it's now been out since June 2016. It's been ported to many systems and had a major piece of DLC released as well. Even though developer Yacht Club Games is hard at work at the new piece of extra content (Specter of Torment) and a Switch port of their flagship title, it looks like that's not all they have plannedThey were asked directly on twitter if they would ever move on from Shovel Knight and they replied with: 

By the sounds of it, this 'new project' has yet to begin development and it could be another year until we hear any specific details. But with the legacy the company has made with their incredibly popular indie game, it's hard not to be excited for anything new they might cook up. At this point the possibilities are endless.