Let's look at recruiting factions as allies:
First, a look at factions and our best bridges into them:
Council/Citadel: Anderson (not garrus, who left)
Turians: Garrus
Krogan: Wrex, Grunt
Asari: Liara, Samara (or the "other" samara, as I try not to have a spoiler)
Salarian: Mordin
Mercenaries: Zaeed, but didn't we pretty much destroy every faction in ME2?
Geth: Legion
Quarians: Tali
Cerberus: Illuminated man, Miranda (try not to spoiler here, but this one may not be woo-able, no?)
Alliance: Anderson, virmire survivor
Drell/Hanar: Thane
SB: the SB

Underworld: Aria
Terminus: Aria? Otherwise, who?
Rachni: The queen
Anti-alliance humans: ?
So, looking at this list, what can we learn? We can eliminate some as simply not making any sense. Through Legion, we are already in with the orthodox geth. Garrus isn't exactly super-connected to the Turian Hierarchy. Liara already had her star turn. Thane is dying: if he reappears it will be as an NPC. Also, his ally-making utility is completely down to who exactly were the Hanar who trained him, and how well-connected they were. Tali's story is too big. Zaeed's story is already told. What exactly would an Anderson mission look like?
So this leaves a few possibilities:
The Salarians: A Mordin mission. One could see the quick-thinking salarians as being the alien species most likely to take action based on scanty or circumstantial evidence, at least action in STG form. Mission could be covert info-gathering with the STG to persuade the Salarians to get on board.
Clan Urdnot: already discussed elsewhere, ad infinitum
Asari: would have to be through Samara, which seems a little questionable politically. "The Justicar Code is so strict and strange, while deciding on war with an enemy which may or may not exist is a much more nuanced issue..."
Cerberus: would be a good choice, but would only make sense for a paragon shep. What would this consist of for a renegade shep? They already love you. Look, one of the major issues of ME3 is how shep relates to cerberus/illusive man. These issues are far better left for ME3.
Alliance: Somewhere this is going to have to be addressed, but I am not sure that Shep simply standing up and saying, "what up, ****es? I'm still alive!" having already rejected the illusive man wouldn't be enough for the alliance to be in his camp.
Underworld/Terminus: Aria. This might be the simplest DLC to imagine. A valuable faction-ally. Easy to work in some good fights. An interesting badass character that everyone likes. Shep helps her with something. She throws in with him.
Did I miss anything?