Let me begin by saying I thoroughly enjoy AC:R's Multiplayer. To me, it doesn't get old. I enjoyed Brotherhood as well, however I can definitely point to many improvements in Revelations that made it a much better MP experience.
The game still could use some improvement, however. Here is my humble opinion of how to improve or fix multiplayer in AC:3.
1. For the love of god, tone down offensive smoke bomb usage. This is a cheap, cowardly tactic as I mentioned on another thread. We all know that some of these players who have prestiged multiple times would be nothing without offensive smoke bombing. They just don't have the skill on their own and thus have to rely on this over-powered ability to get anywhere. The reversed detection meter in AC:R made offensive smoke bombing too powerful as an unfortunate side effect. It's a lame way to get incognito and focus bonuses, often combined with poison. Take this away. It doesn't make sense anyway. Once you've dropped a smokey, you've given yourself away and discretion is completely out the window anyway. If you offensive smoke bomb someone, make reckless the best bonus they can attain and disallow focus bonuses. It'll force players to rely more on pure skill.
2. In Assassinate, remove the compass notification that you have been locked. It's been pointed out before that while the reversed detection meter is a great addition overall, another unfortunate feature of it is that it has broken the way Assassinate is played. Once someone is locked, they typically take off and any stalking/meter building is then impossible. If the player is locked, let them try to figure it out on their own, or give some other not so blatantly obvious clue.
3. Make it even tougher on roofer and runners. Penalize their bonuses even further for such obvious behavior.
4. If you release a map pack like Mediterranean Traveler, please find a way to incorporate into the main selection of maps and not as a special set. Admittedly, the $9.99 price tag for it did put me off. However, being a fan of Ubisoft who is loyal to the AC series, I bought it. I was honestly excited to play the new maps and even moreso to play Siena and San Donato again (my favorite maps from AC: B), particularly with deathmatch. Sadly, activity on these maps is always minimal and you're lucky to find anyone playing at all, yet alone in the mode you're after.
5. Finally, as a minor suggestion, include a way to disarm tripwire bombs. Make it a real challenge, but make it possible somehow, some way.
Looking forward to see what others have to say.