These are some ideas I beleive would be very cool to play in the next AC game as well as some new assassin characters.
Time and location: Camealot, the King Arthur legend.
Templars: the saxons
Assassins: King arthur and the knights of the round table
Pieces of Eden: Excailbur, Merlin's staff, apple of eden # 6
First civiliziation appearence(s): Lady of the lake (Juno)
Real world character: Lisa Miles (Desmond Miles' duaghter from a "missing" night as a bartender), Assassin sleeper agent in Abstergo Inc.
Playable assassin in Atimus: King Arthur (first half of the game), Sir Lacealot (from Arthur's death to the end of the game.)