When I think about it, I guess I kinda agree with you - but I'm really liking the look of Revelations from my point here.
Sure, it's not a big story, but neither was Brotherhood. It might not be the best story compared to the other games, but that's mostly because you already KNOW the character and his motives. As he says in Brotherhood, Ezio intends the Assassins to live on where he cannot, and Revelations is both the drawing together of plot threads and the conclusion of Ezio's story as he finally reaches his goals. Some stories are better than others.
Assassin's Creed II, Brotherhood and Revelations have been Ubisoft's attempt to create a giant plot which follows Ezio's growth from someone seeking revenge to a true master - from a state of no family to one once again. His tiredness shows and, happily for you, this is where his story ends.
Speaking of his story, he's been around for about 60 years. When you think about it, how much change can be put into a game, or rather trilogy, when all the time that passes is 60 years? It's not the same great leap of a time period that it was between Assassin's Creed I and II, so there's not really the sudden invention of crossbows and guns to draw a contrast between the games within Ezio's trilogy. While some of the objectives you do, like the tower defense, might not be too pleasant - isn't it all just a way of trying out new things within a limited schedule? I have no doubt that Ubisoft will avoid using 1-year development schedules past this, since they no longer have the pressure to finish Ezio's story.
Edit: I forgot a point there, about the Templar attention point. To me, it kinda makes sense - sure it's a troublesome mechanic within the game to say the least, but we all know that by 2012 the Assassin order is on its last legs with only a few more pockets of resistance. As far as we can tell, the Assassins failed to embrace the new age like the Templars did, and they've paid for it. Frankly, it makes sense for Ezio's presence in the city of Constantinople to recieve more publicity within the Templar circle - they've heard all about his exploits within Italy.
Ezio is a master assassin now, so it makes sense for them to be trying to find them so Templar growth can begin anew. It's just as easy for them to plant agents within the city, if not easier, as you do. If they see you getting a foothold, then why would they just sit back and allow that to happen? Assassin's Creed is coming to look at the big picture, and that shouldn't just apply to the Assassins but to the Templars too.
And to revise my point about the gameplay changes; Some are going to be hit-and-miss, others won't be. Ubisoft has to balance between locational and timeline gadgets, and I feel they've done that well.