Erm...so I've been thinking about this idea for a while, but I'm afraid to write it on my own just yet.
Idea/plot bunny?
In Pete's World, the 9th Doctor met Martha [where up to you] since Rose didn't exist. Martha traveled with the Doctor eventually ending up in a relationship with him. They have a child that gets hurled through the void during Doomsday. Now the child wouldn't end up appearing directly after the 10th Doctor and Rose parted, but after Martha and the Doctor parted ways [up to you when]. The child regenerates from an older age into a four year old [age can be changed] due to injuries and being in the void for a while. (Yeah, the idea for the regenerating into a four year old came from Mels mentioning that she had changed into a toddler in the middle of New York.) Now how the child ends up in Martha's care? UNIT.
How does the Doctor and the Jones Family react? Up to you.
(Personally I see Francine introducing everyone to the Doctor's 11th body at that point.)
Go crazy, develop whatever kind of plot you want. Just...let me know that it exists if anyone decides to write it.[And I hope no one minds that I posted this here...I wasn't sure where to post it.]

chipper
bouncy
creative