So I think that Ubi could go anywhere in the timeline for the sequal. The future could work in these ways:
1)weapson: assassins use guns these dyas such as sniper rifles and silences weapons. You could still use the blades but it would give you the choice to mix the two or use them separetly.
2)getting around: there's always hanggliding and base jumping, maybe even parachuting from a stealth plane to an objective. Also as said by Swenglish, a grapling hook would be totally perfect. (check out Mirror's Edge, the free running is amazing)
3)story: finally the story. the guild can be looking for these artifacts al over the world considering there's still at least a dozen left when Creed left off. As you search for these artifacts you meet up with other guilds around the world (there have to be others right?) and uncover some hidden conspiracy or somehting like that. Myabe even a war between other guilds, some guilds having already uncovered their own artifacts.
Then there's the possibility that the story will continue immidiatly after Creed.
1)weapons: still the same
2)getting around: smae
3)story: if you read the manual, you'll notice that umm... I can't remember what the decendants name is (I'm fealing Damon...)... anyway, you'll notice that he's not the first of the test subjects, he's like, #17 or somehting. So maybe you could play as another test subject as the templars continue there quest for the artifacts. You could be maybe play a few centries after the crucades as technology slowly "grows" and the templars grow even stronger.
P.S. If you've beaten the game (probably have) then when you get off the animus, walk to the left the the wall, turn a 180 and use eagle vision to look down. There are the numbers 122812 or 12282012, something like that. I'd like for anybody to conferm my hopes that this means that Creed 2 is coming out on December 28, 2012.
I hope that I took no ones idea, if I did, sorry, I didn't read the full forum...