It is twaddle, but I love it too!
In the next series, I think Matthew and Mary will finally come to their senses and plan to marry....BUT.... the Lord and the maid will continue their liaison, she will give birth to his baby son. The baby becomes heir, disinherits Matthew, and Mary will decide that she doesn't love a poor Matthew as much as she did a rich Matthew, so calls the wedding off.
Meanwhile, Sybil and the chauffeur have a daughter in Ireland, but his political leanings eventually result in his involvement with republican forces and he is killed in the Irish War of Independence.
Sybil and her daughter will return to Downton and eventually, the daughter will grow up and fall in love with the maid's son, without realising that he is her uncle.
One more series won't be enough will it?