You could try to give the shadowing/shadowed_by ability to some of the barbarian factions. Like:

faction gauls, religious smith
shadowed_by germans
denari 5000
...
faction germans, balanced mao
shadowing gauls
denari 5000
Basically, If a gaul army revolts, it won´t be a rebel one, it will be a german one, if germany isn´t extinct.