As we have seen from the very well done Alemannia TL, Julius Caesar would have to conquer at least to the Vstula or he woud have the same problems he had stopping at the Rhine. And he would need to conquer Jutland and the Danish Islands and probably the Heruli, who would be very difficult. In fact Scania would be a big headache for subsequent emperors, even though the silver mines of Telemark would be a big attracktion.
Plenty of silver for the Empire in Marcomannia and the Hartz Mountains too. Beyond the Vistula, of course, would be the Venedii and the Balts (the Gepidae, the Litvi, the Kur, the Liv) and the Fenni. Above all the Fenni. Who as other listmembers have noted have many different languages that indo-Europeans can't understand. But will increase in population as their contact with civilization increases.
So instead of Germanic invasions when the climate turns bad, the Romans will get Suomic, Letti, Liv, Kur, Litvi, Slavic, Mordvi, Mari, Komi, Kareli, Udmurti, Bjarmi aand maybe Khamti and Mansi invasions along with the Huns. And maybe even Saami (Lapps) and Norge vikings and sverre for good measure. Ha Odinn! Hakka Palle!