As a series Assasin's Creed follows a central storyline that moves forward in time, as it gradually catches up to the protaganist "Desmond" in the game's "present". Barring spin offs like the New Orleans portable game, the core series by definition requires things to move forward, and also include people who could plausibly have been ancestors of Desmond and locations that could fit into the plots of the Templar/Assasins/etc.. Desmond has a complicated ethnic lineage which is part of the game, but you know what he winds up looking like nowadays so as we move forward distinctive non-white ethnicities become less and less likely as they would have shown a greater effect on what he looks like (a point I'm not alone in bringing up, though it's not popular).
As spy novels and fiction demonstrate hand to hand combat, especially when combined with stealth, never entirely disappear as a valid form of combat, especially on a personal level. Having extremely powerful guns availible in the world doesn't prevent guys from James Bond or Jason Bourne from doing a lot of their fighting up close and personal. A World War I or World War II setting isn't impractical. We've moved Desmond's complicated family tree (since it's all based around genetic memory) to the US, so anything the game does at this point is going to have to feature a way for a US citizen to be involved. With increasing lifespans as time moves on, we're also going to probably wind up seeing larger jumps in history, and more direct connections between protaganists, since we're no longer to really use "distant relations" since the puzzle eventually filters down to one guy, and it becomes narrower as we reach the game's present.
I'm guessing the next steps will either be the US Civil War or Mexican-American War or one of the World Wars as the next step. They want to keep the series going so I'd imagine they want a stopping point between "The American Revolution" and "World War X". They also have to be careful to avoid offensive portrayals as the closer they move to the present the more they are likely to risk POing people based on figures that while old are considered more closely relevent than what they have dealt with before. Ubisoft has already raised some eyebrows, but even they have to be aware there is a point where pushing the envelope could go too far, especially seeing as the reaction to certain portrayals might not be entirely expected. Discussing "The Alamo" for example can open up all kinds of surprising discussions that you might not consider especially given current politics between the US and Mexico.
I'll also mention there has been a recurring rumor that Ubisoft long ago planned to avoid a lot of the upcoming issues if the series went on this long and became successful by trying to move it to the future. Instead of catching up with Desmond, playing one of his descendants and making what happened to Desmond and the answers he might have found the new focus for the games. At one point I remember running into alleged art and story assets showing what was supposed to be a black descendant of Desmond, living 2 or 3 generations ahead in a cyber-punk type future, with pictures of corperate towers with the templar logo (conceptual outlines), and notes about optical camoflauge, monofiliment knives in his shoes (as opposed to a hand blade), and oddly enough exterior lighting systems on the back of those shoes like the old fad (which I seem to remember got some WTF and racist comments). It was probably all bunk, but I tend to think of it whenever I contemplate the future of the series.
I'll also say that the limitations I mention above largely apply to a numbered sequel, which is the central story. In theory you could do the story anywhere in a spin off, by having more templar propaganda films (like the recent portable game was supposed to be) or whatever else. A game set in The Ming Dynasty, or having an Assasin fighting alongside The Foreign Legion (or against them), or whatever else is all possible, as long as it wasn't added to the core series which follows a very specific storyline and progression. As it gets closer to Desmond's timeline the possibilities become increasingly limited, without skipping over him and going generations into the future.