Currently Airing Anime

 Spoiler

If you put it in that sense, I'm glad Mami died, however, I really wish she played an even larger role in the series. She was probably my 2nd favorite character in the entire series, since she always gave out that "pro"-status vibe ever since the first couple episodes. I really don't think she'll ever make another appearance in the series, unless Madoka makes a wish that [blah blah blah]. But enough of my if's, because that's not getting us anywhere.

Here's what I don't understand. Kyubey does all of the contracting for Magical Girls, right? He said he didn't recognize Homura at all, so I'm pretty positive at this point that it's she aged herself, like how you stated, Spidey. Also, because he didn't recognize her, it could mean that she's one of his future contracts, so he hasn't met her yet in this actual timeline (before this Homura time-hopped to this timeline).

How Homura reacted and cried really makes me wonder about her relationship with Madoka, I'm leaning towards the possibility that it's a close friend of Madoka that she met after Madoka became a Magical Girl in the original timeline. This would make the best sense for me, because imagining Homura being Madoka's mother is a bit . . . too much. Meh. We'll see how everything goes.

Theories:

a) Mother, younger because of time-manipulation powers (from another timeline)
b) Close friend she met in another timeline
c) Cat?

 Spoiler

We don't know if Kyubey's the only one. For all we know, the black cat is a contractor as well, and possibly has different terms of agreement. Kyubey never said if he did or didn't contract with Homura, but he was cryptic when Kyouko asked him. If I recalled he said "maybe I did maybe I didn't" perhaps implying he knows that he did, but certainly not in their own timeline. Don't think it exactly points to her aging herself though, she could easily be from just a few weeks into the future, not like she'd need to alter her age to make herself seem like a teenager.

Also, you're missing the lover possibility! How could you when this series has so many yuri shippers watching it!!?!