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OTL, there were large areas of the Roman Empire that could have supported Latin settlement, or developed their own Romance languages, but didn't. This includes broad areas like:
England
Northumbria/South Scotland/Between the Hadriatic and Antionne walls
Vasconia
Egypt
Anatolia
The Levant
Mesopotamia
Other areas in Europe where Latin was widespread but died out
Greece
Your challenge is to have Romance languages spoken in as many of these areas as possible. My take is to perhaps have the WRE be far more prosperous and powerful than the ERE through whatever methods, and then the WRE basically has to go save the ERE, and thus the power of ERE and Hellenophone Rome is crippled, meaning that Latin spreads into the east.