It failed in the real world and it didn´t turn out Barbarian. Even without their Empire, the Romans still civilized Europe (mainly through the spread of Christianity yes but still, it was Roman in essence).
"Barbarian-Barbarian culture" Example:
Alaric II makes peace with Clovis and goes on and does his own thing in Hispania.
Suddenly, Theodemund, king of the Suebi, burst out Gallaecia completely without warning, conquers Hispania and banishes Alaric to Gaul in ever-lasting shame.
Option 1: Theodemund converts remaining Romans and Visigoths into Suebi-Goths/Suebi-Romans and then fully fledged Suebians.
Option 2: Theodemund thinks "Hey, the Visigoths are cool, lets be like them" and converts himself and his people to Visi-Suebi and then Visigoths, creating two Visigothic Kingdoms (provded of course that Alaric hasn´t been ass-stomped by Clovis). He leaves the Romans alone (maybe even giving independence to... something Roman)
Option 3: Theodemund Barbarianizes the Romans too, into Suebi-Romans, then Suebians
Option 4: Theodemund Romanizes the Suebians, leaves remaining Visigoths alone
Option 5: Theodemund Romanizes all of Hispania
That is not what I suggested AT ALL. Like seriously.Option 6: Theodemund romanizes the Suebians and barbarize the Romans, making a Romano-suebian culture, and later on romano-suebianizes the Visigoths and become Portuguese culture
I liked how Mr. Capitalist sees it. You should think of the melting pot in three stages. When two cultures mix and when these two cultures mix with another culture. Depending on the combination, it can lead to real world cultures. Locality could also influence it (as in Léonese appearing in Léon as a product of Visigothic, Suebian and Roman union, or Castille with Visigothic and Roman after a while). Some cultures, like the castillian example I gave, could evolve naturally over time if maintained in a certain frame (as in visigoths get together with romans and become Romano-Visigoths. Then, after a while, they become Castillian.), kinda like the Norse.
No no no no.
I understand the need for minute detail, and I understand the need for infinite ahistorical outcomes where every flap of a butterfly's wings yield an infinite array of histories and possibilities... but this is a game. A game mind you that represents 600 years of English linguistic and cultural change (some of the most drastic in history) in two stages: Saxon -> English. At some point you need to lump things together.
My actual suggestion was to use the divided sub-roman cultures as bases - e.g. Gaulo-Roman, Ibero-Roman, Italo-Roman, Brito-Roman, Afro-Roman - as "bases." They will represent future Romance-derived languages (French, Catalan, Italian, &c as well as two hypothetical ones: Brithenig and a new one for Africa). In order to shift from "ancient" or "vulgar" Latin to the more "modern" cultures they need to come into contact with some sort of Germanic culture. Any Germanic culture will work: Saxon, Goth, Lombard, Suebi, Norse, Frank or possibly a Celtic culture for Gaulo-Romance and Brito-Romance.
So this is how it would work:
A Germanic (or Celtic in special places) ruler conquers Romance lands.
Is he Pagan? If so he falls under considerable pressure to convert to the Christianity (he might still end up a heretic).
Is he Christian, but not the same as the people? If so he falls under considerable pressure to convert to the proper branch of Christianity.
Is he Christian, the right type of Christian, and still Germanic? If so, he triggers the appropriate melting pot to start.
Now in between spaces he can attempt to fight back depending on the size of his nation. This way Saxons will move more Saxons into Britain and wipe out the Brito-Romans - because their Kingdoms were small and homogeneous. But they should still eventually Christianize because melting-pot is the last step, so by the time he coverts either his people or himself to the "right" form of Christianity he has extinguished the Romance culture.
The last loop is modernization. Starting around 800 or 900, any of the remaining Romance provinces will shift to surrounding cultures as Roman influence is dead. The exception is perhaps if a large, stable Kingdom remains purely sub-Roman, perhaps they should shift into a more pure "modern" Roman culture.
So here is an example. I am playing the Lombards, I am pagan, and I invade and conquer Italy (currently "Roman"). My Christian subjects are unhappy being ruled by a pagan, so I let them be and give concessions to allow freedom of worship. Eventually, though, I figure I can convert to the local form of Christianity and gain not only the loyalty of my people, but also boons from the Pope. It is too good to not take, so I convert.
So now I am a Lombard Christian ruling Roman Christians. As my rule continues, more and more Germanic influence filters from the elites (my court and my family) into the laymen. They aren't speaking Lombardic, they aren't suddenly becoming Germanic - they are becoming less Roman. This triggers the melting-pot. Provinces begin to shift from Roman to Italian. Eventually my Kingdom is Italian, my sons speak Italian, and frankly other than my stubbornness I am Italian too, regardless of my actual first tongue. The pattern is complete.
In 900, any left-over Romance provinces shift to the culture of a neighboring non-Roman culture, or the nearest melting-pot culture. So if there are still Gaulo-Roman provinces in 900 and are not controlled by a Gaulo-Roman ruler with nation size 40+ (or something like that) they convert to French or Goth or whatever. If there are (and it should be unlikely) they shift to a secondary fluff culture that is essentially a copy-paste of Gaulo-Roman, but loses the hyphenated name.
Now the primary difference here (from previous suggestions) is that Brito-Roman ALWAYS becomes Brithenig. Gaulo-Roman ALWAYS becomes French. This is an abstraction because the alternative is a multi-step process that is essentially a giant matrix of possibilities. I know it seems shallow, but since it is straightforward and works the same way for each culture with only a few exceptions, you can essentially copy-paste the events for each individual culture.