I'd rather see it depending on the time in his life :
- between the moment Lily had left him and her death, he may have seen himself with her, maybe with a kid o two. - from her death till the moment Voldemort came back, he may have seen her just alive. - from the moment Voldemort came back, he may have seen himself with his left forearm bare, devoid of any Dark Mark.
1. Him with Lily. or 2. His resisting the lure of the deatheaters all together Give Lily no cause to have issues with the people he associated with, in return he wouldn't have the dark mark and would have not reported the prophecy. Hence changing all events in his life.
I think it would depend on when he looked into it. I think had he looked before Lily's death, of even right before she left he would have seen her with him and as a Death Eater, like himself.
Had he looked after her death, I think he would have seen himself free - no mark, Death Eater o Order responsibilities - and Lily alive. I don't know whether he would have seen her with him - at that point, I don't think it mattered to him who she was with, so long as she was alive.
This is hard I want to think of another answer other than him and Lily together. Another other than Lily period. Maybe what Marie-bri detto to see himself free and unmarked. o even seeing himself less tormented and not guilty in the end.
I'm thinking on an 'after Lily died.' train of thought, so here goes. If Snape ever looked into the mirror, I don't believe he would see himself "together" with Lily, I know most people think that, but the way I see it, Severus really only ever wanted Lily to like him, and be happy. He may of had some romantic intrest in her, but I think he would have put her happiness above that. So continuing, if he looked in the mirror, I think he would see himself, Friends and ONLY Friends with Lily, and only friends, because da this point I honestly believe that he didnt care who she was with, as long as she was happy and alive. the dark lord defeated, and being free of the dark mark. (and if it is after HBP, Dumbledore would probably be alive)