If I'm to understand correctly, AC3 is supposed to be closure to all the current storylines for Desmond and the whole "end of the world 2012" thing.
Any games beyond this one probably won't feature Desmond anymore. Think of this as the MGS4 of the Assassin's Creed series: provides closure to most running storylines, and any future games in the series will probably be prequels and/or gaiden stories.
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TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.
"You have issues." - MWC. Pot. Kettle.