Please god somewhere without guns. If they make an AC game focusing on guns instead of melee, I will be very, very sad. Note that I mean focus, not inclusion. The pistol in AC2 and Brotherhood is pretty much fine - limited ammo, long charge-up, not nearly as useful as melee weapons in most encounters.
Now if we want to avoid guns, the present would require some HEINOUS sci-fi nonsense to prevent guns beating every assassin everywhere. The DNA memory thing is already pretty ridiculous, but hopefully they aren't willing to go full-on present day energy shields or anything like that. And I will cry if they make assassins capable of dodging bullets from automatic weapons.
The future could work with some Dune-like technology that protects against projectile weapons, but not melee weapons. I could also see this leading to some pretty righteous free-running among skyscrapers towering miles over the ground or space stations or whatever. Really, they could do whatever they wanted, freed from the necessity of historical accuracy. Wouldn't be too hard to fit into the story either - find another alien artefact thing with this one allowing Desmond to play through a warning of what could happen in the future if he doesn't prevent the catastrophe discussed in AC2.
Failing that, I'd also go for London in some time period between AC2 and present day. Early enough that there wouldn't be any automatic firearms and my understanding is that military-grade firearms would still have been remarkably uncommon in London (and would surely attract too much attention). And I honestly wouldn't particularly mind an ability to dodge SINGLE bullets - that would fit well with the super master assassin thing AC has going on and would be a nice new mechanic. My only problem would be using a similar mechanic to dodge AUTOMATIC gunfire, which seems both patently absurd and slightly outside of the level of unrealism the AC series is usually willing to put up with (a level that I tend to like).