I think it's not so tough to add a multiplayer side feature, like Assassin's Creed Brotherhood had, so long as you don't consider it a primary part of your single player franchise game. There shouldn't be any missable story, basically, and resources for it should only be assigned after you've already allocated everything you need for your perfect as possible version of single player.
Basically, you could take your base stats as new characters into a multiplayer arena style system, design your own soldier, and play minor parts in large battles that are referenced in the main single player story. Your characters get one or two powers mappable at a time, one or two weapons, and then you get upgrades that you can apply to your MP characters' limited slots. That's how I'd do it.
I generally don't play multiplayer games, especially shooters, but having it in the game wouldn't bother me. I could get a few hours out of it. What would be an issue is if some part of the single player game wasn't as expanded out and polished as it should be, because of too much focus on the multiplayer side game. If that's not an issue, all power to 'em. If it's true, that's probably where a good bit of that extra year's developement time was spent, right next to extra polish and customization.