Looking back at Daggerfall, one of the major changes between that game and Morrowind is that there was another race introduces to the series(yes, I know the Orcs became playable, but they existed before that) and that race was the Cyrodilic, who make the core of the Empire culturally. However, how would the Empire be if the Imperials weren't introduced? Just a topic I wondered about and would like some thoughts on.

I doubt the merethic era would have ended in that case.

If you mean what was the empire supposed to be like in Daggerfall lore - it was predominately Nordic I think, but with a huge mix of different races and cultures - a melting pot.

Really interesting question - perhaps the Tsaesci, with their strong connections to the Empire, might have assumed a place at the centre of Tamriel? It was Reman who they stopped for. We might have a very familiar Empire in place, but with Snakemen and half-snakemen instead of Nibenese and Colovians.

But if we're considering the absence of Imperials or the ancestors of Imperials in important historical events, then nothing compares to the Allessian Rebellion not happening. Who knows how long the Ayleid empire could have continued on for if not for the anointing of Al-Esh?

I believe the OP is asking about the presence of an "Imperial race" in the games, not necessarily the presence of Empire-building Men in the universe. As ZindroVokun said, Cyrodiil and the Legions would probably be predominantly filled with Nords, though I'm sure there would be more than a few Bretons as well (Hjalti himself was from Alcaire, though he pretended to be a Nord and gained the Nords' loyalty by turning them against the Bretons that they had kind-of allied themselves with when Cuhlecain became too powerful to fight alone). Cyrodiil, as the Imperial melting pot, would have been populated by almost equal numbers of every race, but the Nords and Bretons--especially Nords--would have been in the highest positions of authority in almost every case, especially in the Imperial City itself. I can imagine Altmer becoming more and more powerful in certain factions (the Mages Guild comes to mind), and even in a city like Anvil that is open to overseas trade and is near Summerset, but the other Elf races would probably have too much stigma to get around (the Dunmer, after all, are basically insect-herding heathens who worship Daedra and hate everyone that isn't Dunmer, so they wouldn't get far; the Bosmer are cannibalistic savages who live in trees and have no proper civilization, or at least so the Imperial revisionists would claim when they don't want to be reminded of Camoran or the later Usurper), and of course the Khajiit, Argonians, and Orcs would never be accepted in positions of authority (except for the Orcs in the Legion; they are, after all, great warriors, and the already-existing reputation and hierarchy of the Legion would allow Orc members to raise themselves up based on their merit and skill. The same is true for the other races, of course, but Orcs are much more likely to succeed in Legion life, and therefore benefit from the Legion's reputation in Cyrodiil as a whole).