So, what if the Frankish kingdom of Austrasia survived the Frankish Empire's dissolution, eventually becoming the nucleus of an independent kingdom?
Frankish sub-kingdoms (essentially Neustria, Austrasia and Burgundy) weren't as much considered as separate entities (as could be, for exemple, Bavaria or even Aquitaine) but as subdivisions of Francia that harboured distinct courts, ruled by a King of the Franks.
Austrasia itself, as a political concept, is not that well attested during the Merovingian period, and mostly as a relatively vague "East" of Francia. The point have to be stressed : Frankish kingships weren't as much seen as a separation of Francia, but a shared kingship whom inspiration was probably to be found in the Late Empire and the shared imperium of Romania.
As such, it didn't formed a distinct "nucleus" on this regard, than Neustria was one century before (very roughly, the rise of Austrasian courts partially come from the decline and collapse of the Mediterranean system in the VIth from one hand, and the rise of North Sea trade).
By the Carolingian period, Austrasia still had the meaning of "Eastern Francia" (to the point, as Neustria could be rarely used for WFrancia, Austrasia was for EFrancia). Rather than trying to resurrect an historiographical Austrasia that may not have existed as such (which doesn't mean that you didn't have a distinct identity, or rather ensemble of familial and regional networks in the Merovingian period, of course), the "easier" choice would be to call EFrancia Austrasia and to leave at this.
Not exactly the most interesting move, I agree, but alternatives aren't particularily easy : you'd need at least a crippling crisis in Merovingian Francia to permanently divises "West" and "East" (leaving the matter of the sub-kingdom of Burgundy for convenience's sake).
That's not impossible, tough : if we use the contemporary exemple of Gothia, the disappearance of the royal dynasty led to an anti-dynastic Gothic kingship where preventing a royal family to re-emerge was a basic institutional feature, provoking several civil wars and disorder, complete with tentatives of secessions (such as the "Eastern Kingdom" of Count Paulus).
We could imagine a similar situation in Francia, with comparable secessionists adventures (to be honest, Gondovald's revolt did looked like this, altough probably not at first, and in the really particularist Aquitaine) leading up to an "Eastern Kingdom" (called Austrasia ITTL, altough possibly with much different borders) to emerge.
How long would it last, and how much the world would be reckognizable, however...
The Early Middle Ages are not an era I really am familiar with, but so far as I can tell the kingdom disappeared largely as a result of the various Frankish partitions.
As said above, Austrasia is essentially a concept, not a determinated region. It doesn't mean it didn't had clear political applications, but the best equivalence I could think would be that you don't use "The Old South" as a complete and precise equivalent of former CSA.
It doesn't help, of course, that the Peppinid/Arnulfid takeover of Francia from Austrasian courts was accompanied with a nobiliar change in Neustria and Burgundy (as well in peripherical regions), as a lot of Xth families there can be traced as being "settled" by Peppin II, Charles Martel and Peppin III.