My 2 cents on RE survival (AH)
To avoid having to actually make up any history I'll just go by the easiest turning point.
9 A.D., P.Q.Varus is thrown by his horse and breaks his neck, L. Caedicius (or L. Asprenas, or anyone) takes command and through following protocol (sending away dependants), or by being careful (trusting Segestest more or Arminius less), or by skill (actual handling of the situation) or by all three manages to blunt our disaster in "Teutoburger Wald" to a mere humiliating defeat, but saves the three legions almost intact.
This leads to Tiberius returning in a hurry, but with lots more forces available and still some semblance of prestige and power the Roman borders are pushed on to Weser and possibly all they way to Elbe.
Then we can return to OTL, our work done
Possibly the future will see three R.E.s instead or two, ERE (Konstantinopelis), WRE (Rome) and NRE (Parisium), but that isn't really important (except in giving the HRE a basis in the NRE, giving it claims on France and Germany as in history, but on England instead of Italy).
The main part is that many, or most, Germanic tribes will become more (or less, but at least a bit) incorporated into the Empire, Romanizised if you will.
So, instead of being invited across the borders fighting for their own kingdoms, they will already be in and want to fight the whole empire (or at least a third).
The 6th century+ dissolution of control might still happen, fragmenting the state and reducing control to local areas as 'limitaires' are organized to larger units to function better (counts) giving us the game situation, but whoever sits in Rome (or Ravenna) will want to call himself emperor, since that is how it is and should be. (Instead of wanting to be king since that is what his ole' man was
(Did anyone understand this ramble or should I try again when I can be more coherent?)