If Denmark will be coming as a faction soon/eventually, I'd like to suggest that the island of Bornholm is included on the map (if nothing else for the sake of historical accuracy, as the island has been there since the end of the ice age), as it was quite important during the 13th century and forward (especially from 1350 to 1450) due to being home to the castle of Hammershus, the largest medieval fortification ever built in Scandinavia. Situated 74 metres (243 ft) above sea level and deemed almost unconquerable during that time.
Historians believe that the construction of the castle began prior to 1255 as a residence for King Valdemar Sejr (Valdemar II), functioning as a base from which to launch the Danish crusades against the pagans of the Baltics. It also served as a home to the archbishop of Lund later on. But no matter who controlled it, its chief function was as a natural fortress securing and restricting access to and from the Baltic sea.
The island of Bornholm itself is located between Denmark, Sweden and Poland, and is characterized by a varied topography consisting of dramatic rock formations in the north sloping down towards pine and deciduous forests and farmland in the middle, and sandy beaches in the south. In most Total War games it is either uninhabited or forgotten, but it seems a real shame to forget it completely. At least if the historical accuracy of the world map is a concern.
Any way you look at it, it is still a pretty significant piece of land, especially considering it had one of the biggest castles of the medieval era on it.
Here is a satellite view of the island and its location in relation to the rest of northern Europe.
Just for the sake of interest, a few images of the Hammershus castle, too. Both in its current condition and images of how it looked before it was abandoned to ruin.
Quick warning: The last two images are pretty large but I didn't want to rescale and upload them elsewhere since then they'd end up broken links a month or two from now.