I think you need an early POD. A fourth century POD is too late in my opinion.
Other members of AH.com have pointed out (in other threads) that an Augustan conquest of Germania with a border at the Elbe River is nearly impossible.
Setting the Weser River as the border instead is ambitous but doable.
This gives you the possebility to have a strong Latin majority along the Rhine when the Empire begins to crumble a few centuries later.
It would also work wonders to keep the area between Rhine and Danube that was lost IOTL to the Suebs.
The OTL Medieval German East Migration origined along the Rhine.
TTL could see a Romance East Migration.
On the other hand:
A Roman border on the Weser might lead to a more densly populated (Germanic) Lower Saxony and furthermore probably to an already German Mecklenburg and Brandenburg by the begin of the middle ages.
But I guess that a strong Romance state centered around the Rhine could conquer Holstein and open the Baltic coast for TTL (Romance speaking) Hansa.
Coastal towns and the major population centers might turn Romance speaking because of the economical bonds with tje Hansa and because of influx of settlers from the Rhine area.
Everything east of the Oder and Pomerania could be majority Romance by the end of the medieval period, with Germanic enclaves in the Harz Mountains, along Havel and Spree and other less favorable places.
With the Industrial Age many of the last Germanics will be assimilated into the Romance speaking population. Religous unity might help with that.
I think your scenario is possible with a 1st century POD.
