Until about the year 500 Proto-Slavic population was squeezed within the Pripyat River Valley until BOOM!, they suddenly and rapidly expanded all over Europe by ca. 600 AD:
One of the most impressive expansions in the history of humanity. It is still a mystery how they managed to accomplish that, becoming the dominant ethnicity of East Europe.
But what if in the 300s or 400s a devastating plague wiped out most of the Proto-Slavs and they could never recover from it. In this scenario the Proto-Slavs are gone before they can expand and capitalize on the power vacuum left by the invasions of the Huns, and migrate across much of Central Europe and Eastern Europe. But the Huns are still there.
In this scenario who would have most likely taken their place over the long-term as the dominant ethno-linguistic group in Eastern, East-Central and South-Eastern Europe?
Not to nitpick, but are you considering Poland and Czechia as "eastern" Europe, rather than central? And is Slovenia and Croatia included in the Slavic lands? If you are I would say east Germanic. If not, perhaps one of the Baltic or Finno Ugric tribes.