There's a ton of stuff they could add. And seeing as I bought almost all the dlc(save for skin packs) for the previous games, I'll probably buy whatever they release for ME3 as well. Except skin packs. Unless they're for like...5 bioware points.
As for ideas!
- It has been said before, but Aria and Omega, yes. You spend huge amounts of time at that station in the second game and it would be nice to have a chance to return.
- Some sort of ending dlc, which Bioware has fortunetly confirmed is in the making. Its a "add more stuff" rather then "fix" dlc, so its to be taken with a pinch of salt and probably wont fix the issue, but maybe it'll help. And its not like they can screw it up more. Right?
- More Elcor! Possibly a mission or two on the Elcor homeworld, where you might also run into Blasto, or some sort of "IRL" version of the movie character.
- One of the most important pieces of dlc I'd like to see would be a conclusion on what happened to the people at the citadel. There's all sorts of rumours and speculation on that currently, ranging from "every single person died" to "A fair few got evacuated in time". Let us play Conrad Verner, as he, perhaps with some sidekick, evacuates as many people as possible from the citadel before the reapers capture it and drags it off.
- Some sort of epilogue content. Bioware has said that this is not likely to happen, but one can still wish for it! Doesnt have to be something big, just something that shows more details on what happens after the flashy explosion. Not like there isnt a need there.
- More maps and more kinds of enemies for the multiplayer. They've already introduced new guns that arent in the singleplayer, so new enemies shouldnt be impossible. Im not neccecerily saying new races, having three should be enough, but maybe bring in a geth colossus, or those squirmy little jumping geth from ME1. Cerberus could get..eh..gunships? And reapers could get harvesters!
Also, perhaps new fighting modes for multiplayer. Im not talking pvp(that would be extremly complicated to add), but maybe they could add more different "special things" you need to do during the co-op fights. Escort missions, summoning a very big enemy you need to bring down in a certain amount of time, retriving objects and maybe drop them off to the shuttle for extract etc etc. There's a lot of interesting stuff they could add for multiplayer.
And a lot more, but that's the stuff at the top of my head.