Actually I’d argue that Chandler and Monica was foreshadowed long before it happened. In the first few years of the show, Monica is frequently shown lounging half-on, half-off Chandler in a way she never did with Joey, and in ways neither Rachel or Phoebe did with Joey or Chandler. Chandler and Monica hooking up was not a surprise to me.
Answering your question, Worf & Troi from TNG.
Well, her old man sure could.
A “kinda, sorta” example might be the Mary Tyler Moore Show. At the start, Lou Grant is married, and was written to be more like a father/mentor figure for naive Mary Richards. Over the course of several seasons, Lou gets divorced, Mary never seems to find a serious romantic interest, and a large chunk of fans began rooting for Mary & Lou to hook up and become a couple.
The next-to-last episode of the series had Mary & Lou actually going out on a date. The end of the episode, they lock lips passionately - and then they both burst out laughing.
Ed Asner said that if he’d written the episode, he would have had them kiss, have Mary turn out the lights…and then out of the darkness the audience would have heard her moaning “Oh, Mr. Grant!” 
Pam and Jim on The Office…I never saw that coming.
On a similar note, Angel and Spike.
…Hey, they did say there was that one time!
On The Wire, Pearlman and Daniels seemed like an unlikely couple (especially since he was married and she was banging McNulty at the beginning). Their relationship was a surprise but worked nicely, IMO.
And in the crappy teen drama category, Joey fell for Pacey on Dawson’s Creek. The even ended up together at the end because the pairing worked so well (and Dawson turned out to be a major douche).
That one always felt forced to me, I never thought they had much chemistry - Apolla and Dualla, I mean.
Sort of the opposite of what the OP is asking, but in the same vein: Turk & Carla from Scrubs were never meant to last, they were supposed to just hook up. But the actors had such good chemistry together, the powers that be kept them together.
And referencing it in the the last episode, with that long, looooooooong kiss…
Is this a whoosh?
Well, in the context of the show, they had known each other longer and had the best boy/girl friendship. But I highly doubt that they ever thought about putting them together.
Chakotay and Seven of Nine from ST: Voyager
Paris and Janeway from ST: Voyager
Wow, you must be more perceptive than I was, because when they came up from under the covers in that London hotel, I was jawdrop floored.
In (I think) the second season, at the end of the episode when the gang went to the beach and Monica got stung by the jellyfish, they actually had Chandler idly suggest that if they never found anyone, they might get together, and she firmly shot it down by saying, “Chandler, I love you, but you will ALWAYS be the guy who peed on me.”
DOn’t you mean Torres? (I didn’t see the whole series, but they did get married, right?)
Chandler and Monica joke about getting together constantly throughout the early episodes, including the time where Monica was in a towel and tells her how much she loves him “as a friend” (that made it into the credits for years afterwards).
As seen on a trip in my Wayback Machine-- Corky Sherwood-Forrest and Grant the producer from Murphy Brown. Also, was I the only one who thought Murp and – oh whatwashisname-- the housepainter were gonna hook up?
Dear sweet Og, now you’ve reminded me of that litter of salamanders they left behind… :smack:
Victor Sifuentes and Grace Van Owen had a thang goin’ on for awhile on L.A. Law, but we all know she was always meant to be with Michael Kuzak.
Pretty sure I’m thinking of Janeway. Something about them getting turned into tadpoles by breaking the technobabble, and then there being baby tadpoles shown swimming away after they got restored.
The thing with Torres happened, too…just wasn’t the one I was referencing.
I saw Joanie and Chachi in Happy Days at the beginning. It never occurred to me that the faux-sibling rivalry would go anywhere, even with Chachi’s puppy crush plot line.
I didn’t know they had ever hooked up! I thought that kiss was them finally getting together. :smack:
Eldon! His name was Eldon.