Well, let's see - what are the key elements of Assassin's Creed games?
- Expansive cityscapes
- Rival factions, most notable one serving as front-of-the-day for the Templars
- Mobility
- Some link back to Altair and the Crusades
Now, one thing is definitely fixed: eventually, Assassin's Creed WILL conclude in modern times and that episode might require a bit of a formula change. Think about it - guards today no longer come running to the site of a murder, swords drawn. They come in cars, they come in helicopter, they use pistols and SMGs. And that's only the beginning. An open world approach like in the first two games will need to be altered somewhat for this to work.
The most important part is probably that we need a sufficiently urbanized setting for this to work. The Italian city states of the early renaissance were notable in this respect, as were the cities of the crusader states. But after that, there is a long time without any of that. Urbanization begins to re-start at about 1830. That leaves us with the following options:
- Industrialized European country (somewhat likely if they are not going for the final installment yet)
- WW1 & WW2 Europe (unlikely as a war setting would be a bit too focused on open conflict and too "twitchy" to begin with for the relevant ancestor to work properly)
- Cold War Europe (don't see that happening, due to it requiring exactly the changes noted above, and if they got those, they could just go on to make the final game in the series already)
- Modern Day (other likely option and essentially what everything has been building up to)