As an experiment, I started a Franks campaign on legendary with the idea of going for a Minor Victory as Rome's ally. Basically, I would ally with the WRE early, then protect the northern frontier by wiping out the Western German, Celt, & Norse factions.
All went fine until around 400. WRE still held almost all of their territories. I controlled Frisia, Scandza, & the Celtic lands. However, by 405 the Alamans, Suebians, & Vandals had managed to wreck Gaul & Iberia (with the Huns wiping out Northern Italy & the eastern frontier). I had let the Alamans do their thing so I could eventually take Belgica from them to fulfill that Victory Condition. It goes without saying that with Rome falling in the next few years I will be Public Enemy #1 in Northern Europe.
Has anyone ever successfully saved the WRE by playing another faction? Seems like a fun challenge and I'm going to try it again soon.