I've been since a month ago with this idea around my head. What about creating a gulf on the southeastern part of the Vinn sea? The gulf would give Esthensia and Vinnmark bigger coastlines, as well as creating a Denmark shaped peninsula where Welstand-Westrijk and Shilesia are, so they would gain a coastline (pretty interesting if they want to fully integrate in the Vinn Sea community) , they would significantly increase their territory and they would lose these artificial-looking frontiers which look really odd in a region that seems to be inspired by Old Europe. Vinnmark's new coastline would look a bit like the east coast of Sweden, with lagoons and pretty detailed islands, and I would be able to connect Berles to the sea (make it look like Stockholm and Helsinki mixed together).
The image features a basic scheme of how would it look, of course we would improve it a lot.
So what do you think?