I do quite like Jo Joyner, and thought her character in series one was a nice one, but I can't imagine them reintroducing her. As someone else suggested, they could potentially sort of reintroduce her by bringing in the ancestor or descendant of Lynda and keep the essence of the character there, but I can't really see them having the Doctor bump into an old friend again (or anyone related to them).
It would have been nice to have had Lynda continue on as a companion first time round to be honest, it's just a shame they had to 'free up' as it were, series two for Rose. I couldn't have seen Lynda carrying on into the xmas special - unless of course she was just this bewildered newcomer caught up in the whole regeneration thing (which wouldn't have really worked that well for the script), and with series two geared toward Rose's departure there probably wouldn't have been much scope to develop her. Then again, it would have been nice to have something else to that series than the Doc and Rose going around, giggling like schoolgirls for much of the time.
I think with this new series, it might be worth them bringing in at least two new companions - because I don't really want to see another one series character come in unless there's more than just the Doctor continuing on. I think the past couple of series (although good) have suffered by being too self contained overall. They seem more like sequals or seperate entities then a continuation. The big reunion of series four's closing episodes didn't really do it for me, and I didn't really see the point of it (although I'm glad it was pretty centred on Donna).
I don't know...I just think it'd be nice to see something different this time, give them time to breathe, rather than saddle the Doctor with a single companion, damned from the moment they profess either their love for the Doc or how they wish to travel with him forever. Martha's rather short tenure on the show did come as a bit of a surprise for me, what with her not being a big name or having a massively publicized departure. I did really enjoy Donna, but it was obvious CT wouldn't be sticking with the show.
I just hope they go back to the drawing board really - back to how it was with the first series, where anything could happen. A new head writer (and maybe a new Doctor) will be good for this.