Celtic western Europe for everyone.
With indirect Roman influence, the tribes would coalesce into larger, more centralized kingdoms over the centuries and in turn keep out or assimilate the Slavs migrating from the east.
Not to mention that we don't know why exactly the Germans started moving in the first place.
in order for that to take place the climate needs to be better, the mass migrations happened because of a climate low (something like the small iceage, 1000 years later).
such an improved climate alone would have massive effects, like the delayed collapse of rome.
Because they retreated when confronted to hunnnic advance.
Climate caused the scandinavian migration in late 8th to 10th century. Not the germanic or hungarian migrations.