While the Fall of Rome is a complex question in itself. Ultimately the failure of the state to protect the frontiers rendered the state moot. Any 'survival' of Rome thesis has to address this fundamental failure. How could Rome maintain its frontier? An early invention of gunpowder would of course do the trick. If it wasn't the Alans, it was the Franks, if it wasn't the Franks it was the Huns, not the Huns, perhaps the Mongols or Persians.
You are absolutely correct about Rome not enforcing its frontiers as it should have. I think there are many reasons for the fall of Rome, all of which I hope to eventually address in my TL. Now I realize my TL is improbable, but I don't believe it is totally impossible or ASB. I didn't make it entirely clear perhaps with my first post on this new thread as far as my POD is concerned, but I did link some prior threads which include the first version of my TL and the beginnings of my novel. Both of these links will help clear up where this improved version of the TL is coming from; especially since it is meant to supplement the novel. If I ever think there is enough interest from this thread for me to justify posting new parts of my novel I will, I'm just waiting for this thread to pick up a bit.
As far as my POD is concerned, it's obviously the failed assassination of Caesar. When I first began writing my novel I looked at what the supposed future plans of Julius Caesar were and I'm assuming that he would have eventually pursued them, had he not been assassinated. Of course him living long enough to actually succeed is all me. But here are several things Caesar planned on doing in OTL:
Draining the Pomptine Marshes
Conquering Parthia
Conquering the Germanic tribes
Just to name a few. Here's what Plutarch in OTL says of Caesar:
"Caesar was born to do great things and to seek constantly for distinction. His many successes, so far from encouraging him to rest and to enjoy the fruits of all his labors, only served to kindle in him fresh confidence for the future, filling his mind with projects of still greater actions and with a passion for new glory, as though he had run through his stock of the old. His feelings can best be described by saying that he was competing with himself, as though he were someone else, and was struggling to make the future excel the past.
Julius Caesar
(Musei Vaticani, Rome)
He had made his plans and preparations for an expedition against the Parthians; after conquering them he proposed to march round the Black Sea by way of Hyrcania, the Caspian Sea, and the Caucasus; he would then invade Scythia, would overrun all the countries bordering on Germania and Germania itself, and would then return to Italy by way of Gaul, thus completing the circuit of his empire which would be bounded on all sides by the ocean.
While this expedition was going on he proposed to dig a canal through the isthmus of Corinth, and had already put Anienus in charge of this undertaking. He also planned to divert the Tiber just below the city into a deep channel, which would bend round towards Circeii and come out into the sea at Terracina, so that there would be a safe and easy passage for merchantmen to Rome [1]. Then too he proposed to drain the marshes by Pometia and Setia [2] and to create a plain which could be cultivated by many thousands of men. He also intended to build great breakwaters along the coast where the sea is nearest to Rome, to clear away all the obstructions which were a danger to shipping at Ostia, and to construct harbors and roadsteads big enough for the great fleets which would lie at anchor there [3]."
http://www.livius.org/caa-can/caesar/caesar_t15.html
Now some things I'm going to have Caesar do (or initiate) in my TL are:
Redefining of Roman defensive strategy and border philosophy (especially the official use of natural borders) - this plays in to what you where saying about OTL Rome's issue with defending its borders
Official colonial policy of resettling Romans from Italy to Roman enclaves in conquered territories
Building of extensive road systems in the conquered areas as soon as possible, completed with a mail carrier system for faster information flow
Restructuring of the military much like Marius before him, with the wide spread use of the Caesar Cipher, both in the legions and along the frontier to help protect the border
Use of the crisis caused by the Civil War and his attempted assissination to fulfill his ambitions of conquest
Massive building projects to lower unemployment and keep the citizens happy
An actual formal Constitution defining clearly the executive (including succession) and legislative branches of government and a complete restructuring (and simplification) of the civil code (which is something Caesar wanted to do eventually)
Gradual citizenship (and rights) for conquered people as an incentive to not rebel en masse, coupled with a strategy of being more open and accepting of foreigners overall
Overhall and simplification of the tax code (something that Octavian did in OTL that made tax collection less corrupt, increased tax flow, and made the tax burden more equitable among the provinces)
In addition many of the above will be expanded on by Caesar's son Ptolemy (Caesarion) and his descendents. And of course not all will go well; Rome will face many, many problems in its long history, but the foundation that Caesar and his first few successors will lay will allow the Empire to endure.