Wait, are you guys saying that a woman who's already played/playing a companion will be portraying the next regeneration of the Doctor? Maybe I'm misunderstanding you guys, but if that's what you're suggesting, how does that work? I know Bond did it with Charles Gray, but... okay, there's no excuse for that.
SPOILERS BELOW (IF YOU'RE NOT THROUGH SERIES 4)
The whole Doctor/Donna thing's coming into play from what I've heard. Because the Doctor can only have 13 regenerations then he was going to transfer his memories and stuff into Donna. So it would literally make Donna a 'new' timelord with 13 new regenerations.
It sounds odd but then again it's sci-fi so it can do whatever it wants. And it's a rumor so take it for what it's worth.