I still wonder it the heir-or-imposter Patrick hasn't an important part to play - it wasn't tied up at all satisfactorily.
I agree that M & M really can't be spun out more as a "will they/won't they" as it will become a complete bore.
However I hope that we haven't seen the last of Mary as I, as a heterosexual male albeit an aged one, find her the most aesthetically satisfying of the Downton ladies by quite a margin.
Sending M & M to the USA might be a good idea, as it would surely increase the already huge American love for DA.
As has been pointed out, the 100th anniversary of the Titanic disaster might point to the resurrection of the Patrick theme.
Not having DNA tests available, any resolution of the matter would probably leave a measure of doubt (the identity of the supposed Russian Princess Anastasia was only finally established once DNA tests were done). This leaves rich possibilities for future inter-sibling and inter-family line tensions, if Edith were to marry Patrick as official "heir" to Lord G.
Julian Fellowes has claimed that he is shown not to be a snob because in DA the downstairs characters are as important as the upstairs ones. I disagree. They may be given equal screen time, but the main emotional weight is still with the toffs.
I hope that JF is not tempted to spin out the Bates-Anna saga ad infinitum, in order to make a point.