My prediction: Most of the DLC will be Multiplayer characters and weapons.
The part for ME3 singleplayer will be the addition of a casino. You will get to walk around and have deep conversation with your crew talking to Shepard about what they think about life and how EDI's tits are smoking hot. Maybe they will throw a bone in for the female players who's Shepard got the middle finger in the first game, and someone well say "Oh I'm so sorry about what Jacob did to you, he was an ass" or "how are you feeling after you watched you boyfriend get stabbed in the gut with a katana and bleed out all over the floor?" The player will maybe get two dialogue options with each conversation that will amount to yes or no or I'm fine or I feel sad, and then Shepard babbles a bunch of stuff that won't change no matter what you pick.
Then you'll hear ambient conversation, people talking about the brutal murders of their children at the hands of a banshee, some guy is going to talk about how he watched a husk rip the baby out of a pregnant woman's belly, some jerkface will say something racist about working with alien races and if the player chooses, Shepard can step in with a big speech about working together, with flowers and rainbows or something.
Then there will be mini games in the casino that will help you get enough credits for all the upgrades and gear on a clean play through.
And finally! The triumphant return of the asari strippers, because that's all the asari are good for and will be remembered for. Shaking their blue butts in a bar for men to drool over and being essentially, male fantasy of hot blue bisexual chicks.