I have come out of "lurking" status to make a prediction. I'm not ever right about these things, but here goes...
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Does anyone know if Elizabeth is still part of the show? She seems to me to be a throw-away character that normally would not have had more that a show or two before she was out of there. I think she is there for something other than just a girlfriend for Ross.
It would be just like the writers to make everyone think that Richard is back for Monica when in fact he is there for his very young girlfriend, Elizabeth. She apparently likes older men and forbidden love. She was seen on national television dancing with Ross. And you know the Chandler/Monica stuff isn't the only thing they will deal with.
I personally like C&M (R&R too) and would enjoy seeing the engagement and eventual marriage. I know Richard just being there is going to cause trouble but I'm not sure it will be as bad as it appears. With one season, maybe two, but most likely just the one left, causing too much trouble is not necessarily the best way to go. Cheating can't be fixed quickly and truthfully I think if they go that way they might as well shoot Monica in the head and bury her in the backyard. it would difinately be character suicide this late in the game. Look what happened to Ross.