Spoiler warning
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All right, I have had a vision of the future, and it has shown me the ending to this year's season finale of Friends. Richard will show up as indicated, and he will be there to throw a wrench into Chandler and Monica's relationship. The vision is cloudy in parts, but I saw Ross and Rachel interacting in meaningful ways throughout the episode. During this finale, two significant stories will unfold. Monica will consider a last ditch effort from Richard to get her back, and Ross and Rachel will consider their relationship once more.
NOW, shift to the very final scene. Chandler is searching for Monica in a hotel that all the friends are staying at (in the area of where the wedding of Chan/Mon is to take place). Rachel and Monica are sharing a room (rather than Chan & Mon, as this will somehow make the wedding night more magical) and a bellhop informs Chandler that earlier he saw a couple enter the room, but he couldn't tell who they were (because he was drunk or stupid or whatever, doesn't matter). At this point there is major viewer uncertainty as to whether Ross & Rachel are going to finally get together again, and the same uncertainty exists as to who Monica will choose.
BACK to Chandler at the door of Monica & Rachel's room. He pulls out the spare key to the room (he grabbed it from the front desk), and he slowly opens the door. The final shot is a close-up of Chandler as he turns on the light and says "OH...MY...GOD!" Then the screen goes black and says: To Be Continued. Chandler's reaction is vague enough not to give any indication as to which couple he saw together in the previously darkened room. And you know what the real kicker is, the show is then cancelled because NBC execs can't come to an agreement with the show's cast. There is no 7th season, and so we never find out what would have happened. This was my vision (although I must admit that I also had a vision that the San Diego Padres would win the World Series this year, and that aint real likely).