Whoever suggested a mission related to Virmire - I think that's perfect. It would help bring in people who started with ME2, but also provide ME1 players with an incredibly poignant way to remember the team mate who isn't there with you, and to help explore some more emotional issues, just as LotSB did.
Revisiting Virmire would allow you to talk about the difficult decision you made there, and could act as a way in to the rift that's grown between Shep and Ash/Kaidan since working with Cerberus. Shep has an amazing attachment to Ash/Kaidan because of the events on Virmire, and going back would be a wonderful opportunity to try to re-establish some of those links.
For the mission itself, I'd love to see it range across different locations, with Virmire as the last location, perhaps. There was an interesting piece of information that was never followed up on in ME1 about Virmire: about the research student who was attacked by those pod crabs (or whatever they were), which was out of character because they are docile creatures. That always struck me as odd, and I always wondered whether there had been attempts at indoctrinating them, as a side project I guess while the geth were on Virmire. Something interesting might be able to come from following up on that.
Perhaps Ash/Kaidan had been following up on it, if they saw a recent report on the case, since they would obviously have a thing for Virmire and it would pique their interest. It's just in council space, so it's not out of the realm of possibility that they'd be able to wangle the assignment, and because of Shep's experience there with them in ME1, and due to the complexity of the case, they call on him/her for assistance, perhaps grudgingly at first.
If this isn't the premise for the DLC I'll weep