3 suggestions in no particular order:
1. For Slavic flags - there is a semi-legendary notion of two tribal unions - Ravens and Falcons, maybe this can be used.
2. Slavic, Celtic, Baltic and Finnish pagans in a coastal provinces should start with decent shipbuilding and ability to raid by the sea. Despite pop-culture claims to Norse superiority, every of this tribes raided everyone else, and Denmark, for example, was subject to Baltic Slavs' raids until 12th century.
3. Slavic women inherited their father's land, if there were no sons. Plus on gavelkind succession, the widow of landowner got part of lands, not sure if this should be implemented, though.
I can try to compile a list of Slavic names for different places by tomorrow, if there's need.