No, in 1066 there wasn't anything advanced across the Atlantic that could invade.
People could think that and prepare for it as much as they wanted, but it wasn't going to happen because it couldn't.
If the world starts as in was on September 15th, 1066, then only a miracle or aliens or supernatural event could allow Native Americans to sail across the Atlantic and invade Europe; an Islamic takeover of Europe or a Roman resurgence are completely within the realms of the real for the world as it was back then.
You're completely missing the point of everything I wrote. We know it couldn't have happened. It's not
historically accurate for it to happen in a game like this (as in, it is not a close retelling of actual history). I'm not in any way disputing that.
But if you want to simulate the flow of history, and what it is like being a leader and the challenges and risks involved in it, then it's just as
historically realistic to have the Aztec invasion as it is to have the Mongol invasions. Or rather (with the way they are done in the game right now) it's just as
historically unrealistic, because both are known beforehand. In real history, you couldn't possibly have known they were going to happen (and sit and prepare for them for 150 years) regardless of whether they happened or not and whether we know now in retrospect that they could have happened or not.
If they had many more possible invasions, and it was completely random when/where and if invasions would happen in any given game, then it would be
historically realistic.