To me it seems that given the territorial extent of the Second Bulgarian Tsardom from the Adriatic Sea to the Danube and beyond, it should've acted as a bridge between the eastern, (mostly?) north-danubian Vlachs and the south-western, south-danubian Vlachs, thus facilitating contact and communication between the early Eastern Romance speakers rather than inhibiting it.
Would this be a pertinent hypothesis or does the lack of as many slavonisms in modern Aromanian discard it from get-go?
I don't think the Second Bulgarian Tsardom had many authority north of Danube. Assen brothers went there to seek help from the Cuman people, and along Cuman people there must've been also Pechenegs and Romanians. The Eastern Romance population was most probably divided, for good, when the Slavs started to settle north and south of the Danube. But while in the north the Slavs were absorbed by the local population, in the south the Slavs absorbed the population already living there (as was the case of the old Bulgars).
Early Romanian language must've been a profound corrupted Latin, mixed with elements coming from the barbarians from the first and second age of migrations; the population could've also been a mix between the various populations that set foot on that territory. When the Slavs arrived they settled north and south of Danube and they also divided themselves, the Danube being a natural border.
The South Slavs managed to create powerful states south of Danube, while the Northern Slavs probably created some sort of political entities, but not as powerful as the ones in the south, as reflected, for example, in the political entities mentioned in early Hungarian chronicles.
Again, the South Slavic population managed to absorb the native population, in some degree, while in the north, the Slavic population was to be absorbed by the local population, giving birth to the Romanian people, a mix between daco-roman population and Slavic population. The most probable reason why this happened was because the territories north of Danube were more populated than the territories south of Danube.
When the Second Bulgarian Tsardom appeared, the Assen dynasty was part of the South Danubian Romance, now probably quite different from the North Danubian Romance.
The Second Bulgarian Tsardom had great impact on the North Danubian Romanian population. Its cultural influence was of great importance. In my opinion, later Romanian culture, administration, army, diplomacy and official customs took its main influence from this Tsardom, whether it was of Byzantine origin or of Slavic origin.