In writing, the golden rule is,"Show don't tell." While playing ME2 many of your squadmates tell great stories about thier achievements and adventures prior to joining your crew.You get thier resume, why not get to expeirence it...To that end...
DLC: Alternative Playable Characters: Miranda/Jacob saving the Citadel from Batarian terrorist.
Archangel and his team on Omega, kicking the Blue Suns, Bloodpack and Eclipse ass.As Garrus described to Shepard!
Subject Zero: She destroyed a space station, stole a top secret spacecraft, was a pirate, and did other stuff, that sounded pretty interesting.Her DLC could be akin to Force Unleashed...
Samara: Her 'cat and mouse' game with Nihilus, and traversing space, helping the helpless.Like Qua Chang Cane of Kung Fu.
Thane Krios: Think Assassins Creed, in space.
A DLC with Dr.Mordin Solus and his STG Team would kick ass! Virmire was one of my favorite missions in ME1, partly b/c of being able to interact and team up with elite Salarian soldiers...
The stories your teammates tell really wet my appetite.They were so detailed that I was sure that BioWare was going to make DLC of them.But so far, this has not happened.But 'never say never'
P.S. ME3, should have a 'Meet the Parents' like DLC, involving Shepard and whomever is your L.I. That would be sweet