IIRC Wallachia in Thessaly was Greek-dominated even during its period of relative independence, so that might be a problem.
Making it a haven for Romance-speaking communities from elsewhere in the Balkans might be one of the few ways, but that's easier said then done. They didn't have any real incentives or reasons to move to Thessaly, and it's hard to think of such an incentive in medieval or early Ottoman times.