Here's what I hope we get, if they're going to give us One Last Rose Story.
For me personally, the best possible thing from whatever we get with Billie this year would be finding out Rose has learned from her time with the Doctor, and has really found a place for herself where she's doing fantastic work in a job that challenges her, and become more than she was, and is ready to say good-bye in a proactive way that replaces the sad little image of the weeping teenager on the beach. Because that was so not the image I wanted to be our last of Rose Tyler.
But I think (hope) they have probably learned lessons from Sarah Jane Smith--we don't want to see our heroes pining for the life they had with the Doctor like lovesick schoolgirls. That's lame. We want to see them living life the way they ought to, with action and drama and relationships and where they are in charge of their own lives, forging ahead. And having adventures.
That's part of what I absolutely adore about SJA. We see Sarah Jane applying the lessons she learnt with the Doctor, but not regretting her circumstances, and instead always reaching for new horizons. Looking back with fondness, but looking forward with joyful anticipation of what new adventures are out there and meeting them head-on on her own terms. We see it with Martha, too. Martha got a fantastic leaving story where she left with her head held high, actively making her own choices, and making the choice that was best for her (and ehr family). Then we see her out there with UNIT and Torchwood, taking the lessons she learned, and still, like Sarah, looking back fondly, but always looking forward.
I want that for Rose. It's what the character deserves--not to be some footnote failed love interest because she didn't Get The Boy And Live Happily Ever After. That's not fair to her, and it's not fair to the little kids who loved her. Or, frankly, the grown-ups. Rose and Martha didn't fail just because they were in love with an alien who was never going to love them back the way they wanted (or needed) him to. They succeeded in so many other ways (granted, I think Martha more than Rose, as far as, alas, Series 2 was concerned), that it shouldn't come down to "but they lost the boy, and the boy was everything." I personally love seeing female leads who don't define themselves through their relationship with the men in their lives. It would be horribly 1950s otherwise, and lame. It's the 21st century, doods. That just simply won't do. Not for today's 8 year olds or 80 year olds.
(erm... yet another reason why SJA pwns so hard, people. Cos it's not important with whom or even if Sarah or Maria fall in love. Not even a little bit. THIS IS AWESOME and a welcome change, as far as I'm concerned. Tho GOD HELP whatever boy tries to ask Maria out, someday. Between Alan, Luke, and Clyde... yeah, that outta be interesting.)
I want to see "Doomsday" as a new beginning--not an ending. Rose got what she'd always wanted: her dad back and her mother happy, mended fences with the ex-boyfriend she treated horribly (and knew she'd treated horribly, which I always liked, even if it was really only explicit in "Boomtown"), and she had a chance to start over and make something of her life, instead of being That Girl With No A Levels Who Worked In a Shop and Shagged The Boy Next Door When She Felt Like It Cos It Was Comfortable And Familiar.
The Doctor showed her a better life: now she needs to live it, and the series (and god help them, some of the fans) needs to let her. For the sake of all those kids who loved her, and will grow up thinking of her fondly as their first companion. And I need to believe that's exactly what she's doing--not pining for what she had, but making her own choices and her own future without the Doctor, and looking forward, not back.
Also, frankly, I want to see what this potentially means for Donna, who is the latest in a long line. Because Rose's story is finished, but Donna's has only just started. And I'm really curious how the two will potentially book-end the first four series of new Who.