The challenge is to have the Roman Empire establish colonization all the way up to the Baltic Sea.
They conquer a part of Northern Germany in the 1st century, establishing a small Baltic coast in Schleswig-Holstein to gain access to a maritime version of the Amber Road.
Honestly, it seems to me that any POD that gets the Romans to the Elbe should have this as a corollary. The Amber trade from the Baltic States down to Italy was a thriving money producer, and if one can get it directly from the Balts into Roman territory without going through Slavic barbarian middlemen, it'll turn a profit.