Sure I'm interested in collaboration, let me know what these names should be changed to.
- Jonava - It more or less covers the areas of historical Lithuanian ethno region of Aukštaitija/Aukshtaitija.
- Wilna - Lietuva or simply Lithuania. It is not perfect, but still close to the borders of Lithuanian tribe and later to the Duchy of Lithuania, Lithuania propria.
- Kowno - Suvalkija. The former part of the Yotvingians lands under Lithuanian rule.
- Grodno - Novogrodok. A very important ancient town, former princedom and very rich and special region of the Grand Duchy of Lithuania. Also several border tweaks and additional Slutsk, Drutsk and Svisloch regions would be nice (later game important Ruthenian princedoms), but there as you wish.
- Bialystok - Brańsk looks much better.
- Brest - Berestia (the ancient Slavic name), Brest is too modern.
- Minsk - the ancient form from chronicles is "Miensk". But you can use both.
- Pinsk - could be Polesia, if you prefer regions names instead of simple towns. Also the border here is absolutely strange: if you check the real map you will see Turov and Pinsk is almost on the same line and quite close. So the border line between them should go rather vertical instead of horizontal like on my map perhaps?
- Krytchaw - Prapojsk is better.
- Brutsi - Lyubech
Also unrelated, but your West Friesland IMO is better as "West Frisia" and Emden as "East Frisia"; Skane as Skania.