While I think this would be great writing, wouldn't this sort of rely on Barney breaking up with Quinn and proposing to Robin immediately? I mean Ted says that after that conversation about Robin's promotion he wouldn't see her again for a while. That means that he wouldn't have seen her between now and said proposal, when he would decide to go off the grid.
There's also the possibility that if Barney does propose to Quinn, Robin is the one who disappears to deal with her feelings for Barney. Whenever the writers referenced the wedding day and what happens, they were maybe implying that Barney marries someone else than originally planned. That, however, seems way less likely than the whole Barney and Robin get engaged really quickly scenario.
Haha, well you have a point and I've got no idea what the timeline will actually look like, but in my head it goes something like this:
Barney gets engaged to Quinn per her essential invitation (and if that girl aint a gold digger looking for a better retirement plan...)
We find out in the finale that, surprise, Robin is his future wife and thus we know Quinn won't work out
Quinn sticks around through the beginning of next season, but then the relationship ends early on
Barney and Robin get engaged during November sweeps (or they get together in November sweeps and get engaged during February sweeps)
Barney and Robin get married on next year's season finale
All the while, Ted has been avoiding Robin until the moment the bride requests to see him
Why might she be requesting to see him after all this time? Well, Ted made this promise a while back to be Robin's best man....and she needs him now.
That all being said, it would be incredibly difficult to keep Robin and Ted apart for that long, and it would also be a little sad considering next season just might be the last. I do like what they're doing with Ted and Robin though because it feels much more real to me than I often see in sitcoms. Being around your ex is not always so easy.