I'm no fool, I am well aware that Tamrielic history is not a carbon copy of real-world history. However it is obvious that the Imperial Empire(s) are directly inspired and/or based off of the Roman Empire, which ruled the Mediterranean for much of the Antiquity period.

Roman History, or you can skip it
At the end of the Antiquity period, the Roman Empire had been divided into the Eastern and the Western Roman Empires, as it was 'too large'. Unfortunately the Western Roman Empire faced many enemies and was overcome by barbarians, native groups, and other tribes that eventually brought the Western Empire to an end. The Eastern Empire did not face these same troubles, and as such they continued to exist for over a 1000 more years. This Eastern Empire would later be known as 'the Byzantine Empire' by many historians, thry were the remnants of the once-great Roman Empire. Where the Western Empire had fallen, many other kingdoms were formed, otherwise known as the Dark Ages, which sparked the Medieval Ages; during the Medieval Ages the Byzantines continued to exist (as I mentioned earlier).

Now on to the Mede Empire
My theory is that the Imperial Empire will not technically 'fall', but it will just decline over a set of years or eras. The Empire once held dominance over the whole of Tamriel, even managing to hold the disease-filled swamplands of Argonia. By the end of the Oblivion Crisis, Summerset Isles, later (re?)named as Alinor by the Altmer, belonged mostly to the Thalmor; the Thalmor would later take Valenwood and then annex Elsweyr. The Empire still had Cyrodiil, Hammerfell, Highrock, and Skyrim; the provinces of Morrowind and Argonia remain unclear to me as to who 'owned' them. After the Great War, tao major things had happened. First the Argonians had invaded Morrowind, and most of Morrowind is now inhospitable. Second, the Great War resulted in Hammerfell seceding from the Empire, and threw Skyrim into a place of unrest.

My Theory is thatthe Imperial Empire will follow in the footsteps of the Byzantines. Much of the Empire will fall, but a portion of it will remain and continue to exist, most obviously in Cyrodiil. In its place, the Empire will be divided into several kingdoms: Argonia, Skyrim, Highrock, Hammerfell, and the Aldmeri Dominion; I am unsure what Morrowind's status is, whether or not it is still considered a 'kingdom' of sorts.I do not believe the Empire will fall, but I do believe it will become a shadow of its former self, and in its place several kingdoms will appear.