Can you expand on this point? I have read that the Roman army along the frontier adopted barbarian styles and war cries, although that's about as specific as I know.
There's probably three things to distinguish.
- Roman army, as it got more barbarized (altough it was already a thing with the heavy use of auxiliaries) tended to use some practices used by Barbarians, such as repetitively hitting your sword with your shield, warcries, "elevating" chiefs on a shield, etc.
- Barbarian influence from defeated Barbarians settled in the Empire (and not just at the border) as auxiliaries/manpower, such as laeti (even if the practice is significantly older) in a first time, and then as foedi in a second time which widespread some Barbarian uses and customs.
- The hegemony of Barbarian identity (which was technically set on a political level in the Vth century before the collapse : you were Barbarian and not a Roman if you ultimately obeyed to a king/petty-king and not the Roman state) led to a problem : as Barbarians which were already importantly romanized and integrating Roman people even before 460's (would it be only as slaves, but that was far from the only way in) AND ruling and integrating Romano-hypenathed provincial elite...Well, it became harder to really spot who was a Barbarian and who was not.
So, Barbarians/Romans pulled several identitarian features such as weapons, histories, laws, clothes and customs that were supposed to be cultural markers.
To give an example, fransiscas were unknown among Franks until the VIth before they looked at what eastern peoples did used, their ethnic history is about how they are actually Trojans, Salic Law is basically the Roman law on foedi, they created Barbarian clothes out of whatever they found Louis-XIV style, and let mustaches grow because Romans thought it was how Barbarians were supposed to look like.
It's not to say that significant and genuine Germanic influence didn't existed, far from it, and it had an important impact on post-imperial Roman society. But there's a lot of stuff that amount to how Romans disguised themselves as Barbarians, with some exception (especially in Aquitaine and Italy). For instance, the city of Rheims rebelled itself in the VIth to not pay taxes, arguing they were Franks and not fully taxable.
I'll add to this that what made the Rhine such a special case is that it was less a barrier than other borders between Roman and Not-Roman administration. It appears to me that its linguistic fluidity between Romance and Germanic related to what administration was in charge and since that flipped a lot so did the lingua franca to the extent the populace was probably highly bilingual. Once the border and administration stabilised so did the language.
I think that rather a stabilised administration (it doesn't seem Merovingian administration was less stressed in Rhineland), it's (at least in a last time) about chanceries and political-cultural-trade centers : until the IXth, these were all set in Gaul, but after Carolingia went down, several (would it be only trough monasteries and the inclusion of peripheries such as Bavaria) were not and especially the prestigious Ottonian centers. One could argue that the growing importance of Frisia as trade center might make this being an earlier process, it's true.
It's probably worth mentioning that thanks to the migrations the Frisians of today are mostly the descendant of continental Angles rather than the probable more Frankish Frisians of Roman times, certainly linguistically anyway.
Giving that you had, at least materially, a great continuity between Rhine and Danemark until the VIth century, even this distinction may not be hugely relevant.
