In other words, Barney could be engaged to Quinn in the current storyline but actually end up marrying Robin (and they could show us how it all happened, next season)/
There is a good chance that something like this will happen. Another thing that I see happening is that they show Ted talking to a few (4-5) women throughout the episode and at the end reveal that one of them is the mother without actually revealing who specifically the mother is.
Really excited for this episode.