So we have The Other Doctor and Rose Smith, lets look into this first.
What Other Doctors do we know of? Theres the meta-crisis Doctor, generally referred to as 10.5, who we assume they mean, but who else could it be? How about Matt Smith? He is defiantly an Other Doctor, but would he be meeting Rose Tyler?
Next we have Rose Smith, with there only being one Rose in Doctor Who, unless you count that Yorkshire Terrier, we assume its Rose Tyler, but married? What other Smiths are there? Theres Mickey/Ricky Smith, but Noel Clarke has walked away from the series to persue an already successful movie career. Then there is Sarah Jane Smith and her adopted son Luke Smith, so a same sex marridge to Sarah Jane? Someone who is almost an OAP? I think not, I can't see Luke going there either. So it has to be the meta-crisis Doctor, who uses the name John Smith when it suits!
So does Rose marry 10.5? Well, she loved the real Doctor, and this is the closest she will ever get to him. But can Rose be pregnant with 10.5s child too? This would be very Russell T Davies!
This is how I'd end the series - 10.5 is with Rose, who is close to having their baby. Meanwhile the Doctor is going to regenerate, but he needs his energy used to create 10.5 back, so against his will, it returns, and 10.5 dies. Not a dry eye in the house, this is very Russell T Davies too!
Then - The baby is born and grows up hating the Doctor, who has caused him to grow up without a father. He has 10.5s TARDIS and is a genius. He reaches say 25, and finds a way back to our dimension in his TARDIS, and through the power of time-travel, this can arrive in season 5 or 6! Compared with the Doctors child, the Master is a pussy! And the current doctor has some real trouble coming!
Of course, it could all be fanwank, and the email I recieved was rubbish, who knows? But I still say it sounds believable!