I think they shouldn't be counting out the Renaissance yet. We have just seen Italy, and we saw it at the turn of century going into the 16th. You have Martin Luther, and John Calvin showing up, you have Henry the VII breaking away from Rome, the Holy Roman Empire beginning to slowly unravel at the seems, and spread through out this century is the French Wars of Religion, the introduction of Catharine de Medici to the French Court and all that it entailed. Plus, they already have a subtle tie in, in that King Henri III (Valois) of France was stabbed by a monk. Jumping the channel to England, you have Henry, then Mary, the Elizabeth, all fun times for England. Also, since some of the glowies on the wall in Abstrego feature South American sites, and those are just being explored in the Renaissance.
Of course I thought they were quick to leave being the Medieval in AC1 as well, although it made more since. If they must jump to another time period though, I personally believe the years leading up to the American Revolution, with the story culminating in the events of the French Revolution would be very worth while. Especially considering how intertwined those two events are.