Just some casual ME3 speculation-
-The whole 'gathering an army' thing seems how this is going to me- but it's not going to be what decides whether you defeat the Reapers. That has to be a Shepard action, like going aboard the 'main' Reaper, fighting indoctrination, battle of firepower and wills, etc. No, the additional allies will (similar to ME2) determine how well/poorly the rest of the galaxy fares in the battle. Casualties, planets razed, etc.
-As for who's coming back...I don't know that the DLC characters will be back; I agree with someone who speculated Samara/Morinth having a low chance for returning...really, most of the potential LIs (except Jacob *sigh*) and Grunt/Mordin/Legion are all fan favorites, so that much is up in the air. But if BW thinks they can handle giving us 14 potential squadmates (15 with Wrex, but he's a long shot).....more power to them.
-Would it not be awesome to have a small, special cutscene near the end of the final battle with your LI? Not mushy, but like "I know you can do this! *meaningful look*" Okay, I'll admit, I have these fantasies of Shep having to mentally face down Reaper indoctrination and Garrus giving her that last boost of morale she needs to fight it.....

So many wonderful ways that could play out. People w/out LIs would either not have it or just have whatever random teammate who was there giving the line.
*Sigh* I need to go to bed and stop wishing ME3 would come out faster....