While destruction or evolution of the Roman Empire is a real historical discussion, to say that it was divided among it's armies and generals is just plain wrong. The Vandals, the Francs, and the Suebi were never in any way part of the Roman power structure. They alone acount for the loss of North Africa, most of Hispania, and a good part of Gaul. The Visigoths had a more complicated relationship with the empire, but I would argue, that they were, from when they were settled in Gaul to the end of the Western Roman Empire, alway as a group politically distinct from the Roman Empire and it's army.
Historical quibbles aside, I think an end date some time in the 1. century AD would be best. I'm actually most interested in the rise (or rather stopping the rise more often than not) of the Roman Empire for a game and as others have said the Pax Romana would make some very different mechanics, to make that time interesting to play and the stagnation/fall/evolution whatever you want to call it, would also be very difficult to translate into mechanics. On top of that the rise of Rome wasn't a sure thing for quite a while, so it wouldn't even be certain, that those mechanics are needed for every playthrough, or whether those mechanics would also need to work e.g. for a successful Carthaginian Empire. The Roman era is best split into two games.
I did say "for the most part" there. However, it would appear that you would be surprised just how many of the "barbarian invaders" were actually "Foederati" turning on their employers.
For instance, there were Franks who WERE Foederati - Julian the Apostate is said to have "allowed" the Salian Franks to settle in Toxandri (Belgium, Flanders region) around AD 358, so long as they protected the empire as Foederati. As Foederati they were technically meant to fight for and defend the Roman Empire. They most likely did so for a time, but it didn't take them long to get a head start on swallowing up Gaul for themselves from their prime position.
The Suebi that you mention is a very broad name - and gets complicated. What tribes were actually Suebi changes depending on what source you are reading. Do you mean the Allemanni, Bavarians, Thuringians, Langobards, Marcomanni (perhaps) to name just a few? There are numerous peoples who were said to be "Suebi" by Classical authors - in fact the name "Suebi" seems almost to have been used for the word "German" at times. With the Allemani, you are correct, as a people they never seemed to served the Roman Empire, were always opposed to it - but that doesn't mean individuals or Allemanic warparties never hired themselves out to the empire. Most of the other peoples were Foederati in whole or part at some point or another, even if one does not usually think of them that way. For instance, in AD 409 a group of Marcomanni, Quadi and Buri were "allowed" to settle in Gallicia , where they served as Foederati and went on to found the Suebian kingdom of Gaellicia as the Empire lost cohesion. While this settlement might just have been an acceptance of Suebian conquest (just like most of the others probably), it is noteable that Suebian Foederati DID fight with Imperial forces soon after against the Vandals in Hispania. So Foederati the Suebians were.
As for the Vandals: Stillicho, the famous "Roman" General, was himself half Vandal. As you know, Stilicho led a "Roman" force that fought Alaric the Goth - another "Roman" general, who was actually a Goth, who led a force of Goths that had been hired originally as Foederati. The Vandals themselves, from AD 406, in alliance with Iranian Alans and Suebians, marched westward into Roman lands, defeating Frankish Foederati in Northern Gaul before storming into Hispania, where Vandals, Alans and Suebians alike were all granted lands and (officially and on paper at least) made Foederati. The Vandals very soon after turned on their allies the Alans, and then the empire, fighting several battles against Imperial-Gothic-Seubian alliances (The Gallician Suebi?) before eventually crossing to Africa. But Foederati the Vandals had been - on paper at least, however briefly.
The list goes on. In most (not all mind, but most) cases of "Barbarian invasion" the forces got so far into Roman lands and then were settled as "Foederati" - if only to appease them and dissuade them from rampaging further. How actively these peoples actually prosecuted their duties as Foederati is debatable. It is very likely that in many cases it was just a way for the Roman central authority to claim that there was no longer an enemy to deal with, a way to deceive themselves that provinces weren't gradually being lost piecemeal. But whatever the reality, officially, Foederati they were. And in their settlement regions they stayed for a time, maybe even served the empire for a time, as it was hoped they would, before expanding to carve out lands for themselves as it became clear the empire had lost it's teeth - from within the empire itself. People after people did this AFTER having been settled as "Foederati".
While this is indeed an overly simplistic overview of events, Foederati forces appear to have made up the vast majority of the Imperial forces in the 5th Century, and many of the later Commanders were of of "Barbaric" lineage too, so when I say the armies and Generals carved up the empire amongst themselves - I am not really too far from the truth. From a certain point of view.