[Warning: mild spoiler for season six premiere follows]
Season six opens with a wedding. We are meant, at the beginning of the episode, to think that it is Ted’s wedding. At the end of the episode, it is revealed that Ted is merely the best man. The obvious inference is that this is Barney’s wedding; probably with Robin. This is far too simple.
I believe there are two possible solutions.
(1) Over the course of the season, we will be introduced to a new character and that character will become friends with Ted and get married; probably to Robin. This would be a let-down which is why I much prefer solution (2) This is Marshall and Lily’s wedding. Wait, what? But Marshall and Lily are already married, you say? You forget, they were married by Barney before the ceremony. I predict that Barney will learn that he was not actually authorized to perform the marriage or for some other reason, their marriage is void. This will happen shortly after Lily learns she is pregnant. Therefore, Marshall and Lily will have to have a second wedding before the baby is born out-of-wedlock.