MW3 will have new content up to September, and the big marketing push for the next title will likely start right when the last content collection is released. Many people will have stopped playing MW3 by that time, and a few others will move to Black Ops in anticipation for Black Ops 2. The major premise for this idea is that adding maps from CoD4 and MW2 to MW3 costs IW/Sledgehammer very little.
I think a great way to give a last hurrah to the game would be to release a map pack of 5 or more maps from old games, and offer a large XP boost to those that buy the map packs. Sell 5 maps for $10 (or even $5) and along with it something like 4 times the normal XP while playing during the last month of the game.
Another idea is to sell Elite Premium memberships with double or quadruple XP incentives. If someone buys a year of Elite Premium between Oct 1st and the release of the next title give them XP incentives in MW3 (or prestige tokens to buy double XP, weapon XP, other stuff). If that were the case I would consider buying Elite Premium separate rather than buying the Hardened Edition of the game as I did with MW3.