Assassin's Creed Brotherhood didn't do much to advance the animus story, but it did a lot for the modern times story.
What was REALLY good about Brotherhood, was the increase in quality of presentation, the refining and additions to gameplay, and side-missions. However, the more substantial side-missions resulted in the actual storyline having only like, 3 assassinations, and many of the new mechanics didn't have much of a purpose.
I think this is down to them not having a big enough team on the game. For Revelations, however, the main Montreal studio alone is 200+ employees, and certain parts of the game are handled by 6+ smaller ubisoft studios around the world.
This means that Revelations will have more content. As well, the game is going to be about giving you a really amazing sandbox to work in, and then punctuating that with cinematic moments, which flow naturally through that, because of the integration of gameplay in them, similar to Uncharted.
The whole design philosiphy seems really great, and the features they're introducing seem a lot more relevant than what we got in brotherhood, and most of the brotherhood features are being refined or consolidated. For example, brotherhood allowed you to throw bombs when locked on a target. Revelations is introducing bomb crafting, which allows you to make whatever type of bomb you want for your playstyle, like flash-bombs, damaging-bombs, smoke bombs, (and more that they haven't talked about), and add ingredients together to minutely change the characteristics of each type. Such as making your smoke bomb have a bigger area of effect at the cost of the time it lasts, or making the smoke poisionous at the cost of it only affecting a really small area.
Ok, now I want to talk about Brotherhood. OP, you call it a cash in, but it was only a little bit shorter than AC2 if you did all the side-content, which was better than any side-content previously seen in AC.
It was magnitudes more fun to mess around in than AC2, and it actually had a significant graphical increase.
It was just plain fun to play. I can't call anything a cash in if I get more than 30 hours of fun out of it. And that's just single-player.
Multiplayer suffered from long times to get a game, and lag, but it is really unique to anything I've ever played. It was about as far away from cash in multiplayer as you could get! Just look at bioshock 2 for that. Bioshock 2's multi was kinda fun, but the polish just was not there, and the lag was incredible, and the controls sluggish.
Then compare to Brotherhood: abilities that are balanced, games that are won not by the amount of kills that you get, but by how stylishly you get them, or by how well you frustrate your pursuer, mechanics that are slick and work just as they're intended.
Brotherhood was made to make money, yes. But Ubisoft said "Hey, if our boss tells us to make more so fast, lets make them freaking awesome!" And now Ubisoft has given them the tools to make an even bigger leap than AC2 to Brotherhood!
Revelations is in Turkey, and Ezio has a totally different visual style. It's not really a change of character or time, all that much, but interesting things happened in that part of the world at that time.
But really, what other series gets so much flak for using the same character for 3 games in a row at different points in his life? 3 is not a lot. Ubisoft thinks of Ac2, brotherhood, and revelations as part of the Ezio trilogy. AC3 will be the third part of the Desmond trilogy, with a new ancestor, setting, and probably some assassinations with Desmond in modern times.
Releasing all these games has given them a lot of practice with implementing features, so they'll be pros at it by AC3.
EDIT: I'm betting he doesn't play as big a role, as in if he's playable at all it's only for a while. I'm probably wrong, but I can't see him being more than a leader unless this happens days after Brotherhood. What WOULD be interesting is if Altair is somehow still alive due to apple overuse and he takes Ezio's son under his wing. I can't see any reason to even hint at Altair having a leading role unless he was somehow actually featured in the main story.
Ezio is on a quest to find seals that Altair recorded his memories animus-style on, and then bring them to Masyaf to unlock a weapon that Altair found. But yeah, Altair's dead. We'llbe playing him through the sequences that Ezio experiences when he finds each seal.
