Kylie would have made a good companion but I doubt she has the time to take at least six months out of her schedule to film a whole series.
I enjoyed the Christmas special this year, but it really needs to be taken in the context in which it's meant. It's a Christmas show, much more akin with a Christmas Pantomime than a normal episode of Doctor Who and I think it works well as that. If you consider it a normal episode then it wasn't the strongest but as a bit of throwaway fluff on Christmas day then it was perfect for that.
I thoroughly enjoyed it.
And for what it's worth, I'm not a great fan of Tennant, I thought Chris Ecclestone did a better job and it's a real shame that he didn't stay for more than one series to be able to develop the character a bit further.