...Here's some suggestions:
Assassin’s Creed 3
Hidden Blade(s)- Remain the same, duel upgrade, stronger metal upgrade (?)
Hidden Gun/Pistol- This could go two directions, either leave it the way it is now OR
-Hidden Thruster- Small device that emits a blast similar to the thruster of a jet killing/wounding enemies in a cone in front of you
Crossbow- Just modify it to be a silenced sniper rifle
Sword- Same (Maybe something like this http://www.2-clicks-swords.com...category_image/4.jpg)
Dagger/Knife- Same (Maybe something like this http://www.marloweknives.com/IKBS1and2.jpg)
Throwing Knives- Same
Money- Same but with paper bills
Smoke Bomb- The same BUT with a Gas Mask upgrade you would be able to have lethal gas that would slowly kill you enemies if they stay in it too long
Medicine- Either some super medicine or morphine to dull the pain
Logically in modern time enemies WON’T fight well in melee range so it’ll most likely be a race to close in on them quickly and decimate them. If that’s the case you may want to have 1 or 2 more ranged weapons to combat them. Oh and a net launcher…. long aiming/prep time but once launched will temporarily keep your target tied to the ground.
Hiding Spots and Interactive Objects
-Leaps of faith would be performed into either open trash cans OR onto shades near building doors (they kind you always see cartoon characters landing on and surviving… I mean if he can land in a hay bale…)
-You could hide in/on: Open trash cans, fountains, benches, telephone booths(?), parked cars, and open sewer entrances
-Bodies could be hidden in all of the places you can besides benches and telephone booths
Travel and Civilians
Cars would replace horses.
Swords and ranged weapons would remain the only weapons you can use from a car.
Driving on streets would keep you hidden but driving over the sidewalk but cause notoriety
Driving into walls head on would cause the car to crash and you to be forced to get out (similar to bodies the cars would eventually disappear)
Civilians will dive out of the way of an oncoming car BUT they can be run over killing them and raising your notoriety/decreasing you synch level (not as quickly as directly killing them though because it would be easy to accidentally run over a couple)
Climbing would still function the same way
Telephone/power lines would be similar to the lines in past games.
Because modern buildings tend to be smoother, see Infamous™ for how Desmond might go about climbing buildings
Tombs, 16 Puzzles, and Collectibles
Assassin Tombs/Romulus Lairs could be replaced by templar labs/facilities, sewers, and landmarks
Billboards with hidden doors could serve the same purpose as the 16 puzzles in AC:B
Since little wooden chests and flags are out of the question collecting discarded templar cell phones (with vital audio info), chasing down muggers, and finding purses could serve as replacements.