Nothing anywhere near this far along in history ever again, that's for sure. Makes for a boring setting and all the guns ruin it. Ancient Rome would've seemed like a lovely idea, but they did kind of already have a game in the city, albeit much later.
I don't think guns ruined AC3, in fact they were handled pretty well.
I would like them to set it during the first civilization era, but that may not happen till AC4, and they'll likely continue Conner's story next year unfortunately. The first civ setting could be such very interesting in terms of architecture and gameplay possibilities. A setting like the french revolution, WW2, egypt of even ancient china doesn't excite me much. I just don't see them differentiating themselves much from previous settings and gameplay. Actually imperial china could be pretty damn coll now thta i think about it. --- The internet: Where the men are men, the women are men, and the children are the FBI Okami! Play this game if you haven't already!
If they don't do the future to address the releasing of Juno in the modern day.. than they would be completely wasting the clues in ACIII; that you are watching Desmond's life from the future with the hints that Ancients built an Animus to look forward. We built an Animus to look backwards. Why else would there be Animus like effects when you play as Desmond?
Assuming Connor's going to be in the next game, the French Revolution. It makes sense with some of the small hints we've received, in this game and Brotherhood. The Phrygian Cap and Masonic Eye were both seen in the end of Brotherhood, the Eye obviously pointing to the American Revolution, which we played and the French Declaration of the Rights of Man and the Citizen is the only document where both symbols are shown together and was signed in 1789 which fits in with Connors life. It makes sense that this be the setting of the next game and it's honestly where I want to see it.
Being back in Europe would open doors for Connor to get more involved in the Assassin Order and play a bigger role in it's development for that time period. This could lead to him really getting things going in America for the Assassins, and maybe some flashback aspects like Altair in Revelations but about Ezio, allowing Connor to gain greater insight into the history and helping him grow and evolve a little, although that may be a little much but if they did it right could work. --- There's no such thing as a Saint without a past, or a sinner without a future