Frankly, I think Ted and Robin AGAIN is just fodder for another half-season's worth of episodes. It's been done to death, and I can see how it COULD have worked if we hadn't already established a thousand times that Robin isn't the mother.
What you have to say, PM, about Robin being a sort of gateway to where Ted finds his wife is a pretty good guess as well.
I honestly don't think the show has much longer to go, Lily is going to have her child, and that will be supply a few stories, but sooner or later it's going to have to come back to Barney's wedding, which is where Ted will meet "The Mother". Lily's Dad also seems to be a bit of a waste of time. I think that in the end, Barney will marry Robin, since they both seem to be outrageously self-serving when it comes to relationships.