I can understand WHY they are bringing the character Donna back even if I personally don't want it. It was ok on a one off episode but I find it hard to see how it can work for a whole series. I am happy to be proved wrong and it won't stop me watching Doctor Who, if only for the hope that her character will soften to how she was at the end of The Runaway Bride. Her character was a refreshing change to Rose but I'll find it hard to stand her yelling and sometimes, over acting. I want her to play a more serious character, yes with some outbursts for comical value but a more serious approach, IMHO, would be well appreciated.
If you look at The Runaway Bride objectively you can see why her character was quite rude, moany etc because that would be a lot to deal with any day let alone a wedding day. Especially after finding out that her fiance didn't actually like her at all. So her reactions can be excused. There is potential to expand her character with more toned down reactions. But I guess we all have to wait for the next series to find out.
I had the same misgivings about Freema as well. TBH I really disliked her in most of the episodes but at the end of the series, especially the last episode, I did warm to her a lot. She came into her own as a character, strong and independant. I can hope for the same to happen to Donna
And as for the rumours that there won't be any more series after series 4, BBC have announed that in 2010 there will be series 5 and in 2009 there will be 3 feature length specials with David Tennant. After that it's guesswork but there is a good chance that unless the ratings go down considerably during the next 2 series and the specials then it won't be axed after series 5. And Rose coming back? Don't get me wrong, I love the Rose and Doctor relationship but why? They closed the doors on that at the end of the new series 2. I can't see how they can be opened again, unless she's still stuck there but they can communicate through the parallel worlds. I don't know, it would be interesting to see how they get around it but I personally want to the Doctor to move on as he has been doing throughout series 3.
I have also heard rumours that RTD wants Richard Armatige to be the next Doctor after David Tennant. I have no idea about the credibility of these rumours (seeing as I have no idea where they are from yet) but I think he'd make a good addition to the cast.