Nope, it would make no sense as Imperator does not simulate the Roman Imperial Period
Which is funny for a game called Imperator: Rome.
I would be surprised if they do
Even if PDX don't, I'd be very surprised if mods don't.
The Roman Empire falls in 1453 though

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Honestly, the mechanics of the game should work to model the era up to about Charlemagne just about right. If Paradox isn't gonna do it, then mods surely will (WTWSMS II, maybe?)
Now, here lies the real question for the eventual mod that extends the timeline up until Charlemagne (or at the very least to WTWSMS, which I am familiar with, but never actually played), is for how PDX is going to implement barbarian warfare for IR.
Chances are they won't touch on the concept of hordes if they don't make an imperial expansion, so modders will have to work that in through events, buffs and whatnot.
Which leads me to biggest issue imo, the dismemberment of Rome (or any big blob that the player would likely form).
I think there's little point in going to a ck2 game where you control most of Europe and north africa, so that's really important.
I think you should have a very small margin of error during those endgame events (without it being actually impossible), be it from a mod or an actual expansion.
At some point before the endgame, when the empire is too overextended, you should be presented with an event/decision to split it in two or more parts, with a choice to play any of them.
The part that has the capital most exposed to the Barbarians would face the blunt of the invasion, but the other part could have a better chance of survival (but not unmolested).
Then, after the barbarians events/hordes, the exposed part would likely fall, safe for a very efficient loyalty and corruption management.
If you're controlling the fallen part, maybe you could receive a decision to evacuate your family ("your" being the current ruler of the empire) to a safer place, which would work as the county where your dynasty would start in ck2.
I'd say it's acceptable to be reduced to a de jure kingdom, preferably a duchy, but even a county, perhaps depending on the loyalty of the governors in the area you evacuate to and your invested wealth.
Then, there's also Christianity, which should decrease the authority of the empire or the loyalty to the emperor, which would facilitate the decline.
Those are just some ideas, it doesn't matter if it's done that way, as long as the result is the same: high difficulty to finish the game with the empire together.