I really don't know much about the Slavic groups who moved into Europe. However I do know they were spreading out westward.
I have an idea for a country, the Germans in what we call Germany are displaced and assimilated into this incoming group.
The area between the Rhine and Oder rivers becomes their homeland.
1. Europe geographically is up to Ural mountains.
2. Slavs are REAL EUROPIANS who was driven from Carpathian mountains by Goths to modern Russia - they reached Kiev, Moscow and Novgorod only in 5-6th century and subdued local tribes.
South Slavs together with Avars attacked Bizantian Empire and settled in 5-6th centuries in modern Bulgaria, Serbia, Croatia and even in Greece.
West Slavs moved from Carpathian Mts to the west after germanic tribes moved to France, Italy, Brittania - after fall of West Roman Empire.
So your idea with would be true if ALL Germans left Rhine and moved to e.g. France - then btw french lang would be more germanic than roman.
But West Slavs have no power to move Germans and germans didn't want to leave their rich Rhine homland - that's why West Slavs had to come back to Oder and further to the east after german "Drang nah Osten" since 11-12th centuries.