A migration of western germanic tribes along the Danube is really unlikely. They began entering in western part as early than III century, and formed themselves really close of Roman Empire; when Goths were migrating peoples.
I wouldn't rule out a germanic settlement in the East, but the west is really a more easy prey :
- WRE was poorer, while wealty, and had limited ressources to resist
- WRE was less populated, and had to use germans to settle some lands and to form armies
- WRE was almost entierly made of one continental piece, when ERE beneficied from having two parts clearly separated from each other and hard to cross as they were able to maintain a navy.
Except if Constantinople falls, and that Germans takeover ERE's navy, I can't see a fall of the eastern part of the Empire. And frankly, if they able to do that, I don't see the WRE having much more chances, as Africa is doomed to be taken over as well.