Continuing in the vein of totally 100% selfishness, I pretty much want every character from Harry Potter ever, but especially the Marauders. I think James would be #1 on Sirius's list for obvious reasons, followed quickly but Lily, but I think a Remus would be amazing since, while Sirius has been made aware of what was ACTUALLY going down, there's still some hurt feeling and distrust there. (Plus, I have ~headcanons.)

Perhaps not a Peter because Sirius would, without a doubt, kill him on the spot. Heh.

I think a Snape, young or old, would also be super interesting, obviously. And Dumbledore and OMG MCGONAGALL PLEEEEASE. Ugh. Just... everyone.