I thought an actress named Michelle Forbes (movie “Kalifornia”, Star Trek Nex Generation on TV) had potential to be a big star, but she seems to be nowhere now. (Star-wise any way). Of course, I’ve never had great predictive powers. Still, how about you all out there - any musicians or actors you were convinced were gonna be the next big thing, or that you wanted to be the next big thing, and that just fizzled?
Julia Ormond. She was a young actress who made a lot of movies with some big-name stars in the '90s: the Sabrina remake with Harrison Ford, First Knight with Sean Connery and Richard Gere, something with Brad Pitt too. Then she just disappeared. I haven’t even seen those movies, but I remember seeing her face and hearing her name EVERYWHERE for a while, and then nothing.
I’m actually waiting for Jimmy Fallon to become a huge star and household name now that he left Saturday Night Live. While SNL fans here on the Straight Dope and elsewhere seem to either love him or hate him, I think he has potential to blow up like Adam Sandler did–and I find Fallon much funnier, more charming, and less obnoxious than Sandler. The fact that teenage girls think he’s cute shouldn’t hurt him either.
Of course, I’m one of the biggest Bruce Campbell fans out there, and it’s a crime that man isn’t a Hollywood A-lister and a household name. He might be in his 40s already, but I still don’t think it’s too late for one perfect movie or TV role to catapult him to long-deserved superstardom.
Some of these have worked regularly, but never had the stardom they deserved, especially after a terrific first performance (in parentheses)
Carrie Snodgrass (“Diary of a Mad Housewife”)
Linda Manz (“Days of Heaven”)
Pamela Reed (“Eyewitness”)
Alfre Woodard (Guest slot in “Hill Street Blues”)
Barbara Harris (“Nashville/Freaky Friday”)
Rebecca DeMornay (“Risky Business”)
If we include musicians:
Marc Cohen
My daughter used to watch “All That” and “Keenan & Kel” on Nickelodeon. I didn’t really care for either of the shows, but I liked Kel Mitchell. He used to mug like an idiot for the camera, but he also has an excellent sense of comedic timing, and as he got older his acting got better. I really thought he was going to be a lot bigger than he is now.
Remember Terence Trent D’arby? I thought he was going to be huge. Nope. And I coulda swore Billy Idol was going to be the next Jim Morrison.
Boy, I really suck at predicting fame. 
I always thought Oded Fehr (Ardeth Bay from the The Mummy) would be a lot bigger star. He did the Mummy movies, Duece Bigelow, and a couple of tv shows, but has completely dropped off the radar. Too bad, too. I think the man is a HOTTIE!
I was genuinely surprised when Bijou Phillips didn’t blow up back in '98 or so. Britney and Christina were at the height of their popularity, and Bijou presented (at least superficially) a punkier, more “bad girl” take on the whole cute girl pop thing - the same shtick that Pink and Avril Lavigne took to the bank a few years later.
Gerrit Graham was hilarious in “Phantom of the Paradise” and “Used Cars” but never seemed to get that role that would catapult him into a comedy A-list actor.
Several of the MST3K crew should be doing major stuff – Mike Nelson, Trace Beaulieu, Joel Hodgson, Mary Jo Pehl … probably a few others as well. They’re much funnier than the crapheads who do A-list comedies … Adam Sandler? Jim Carey? Puh-leze!
I’m still shocked that Happy Rhodes isn’t successful. She should have been at least as big as Sarah McLachlan (I love her too) but without a record company push/blitz, it was never going to happen. She’s still making music, thank goodness, but has to record bits of songs at a time as she can afford it. She’s been working on her 11th album for several years now. She’s not slow, she’s just broke. It’s a damned shame.
I thought Kyle MacLachlan was gonna be huge after Twin Peaks. Instead, he made Showgirls.
I remember watching her performance and just being blown away.
She was also excellent in the highly enjoyable *Heart and Souls * (1993).
Ken Wahl should have gone the way of Bruce Willis and become a major Hollywood star after his role in Wiseguy. Instead, his career self-destructed into a morass of alchoholism, bloating and marriage to the Barbi twins. Go figure.
Damn, his first album was amazing – his version of “Who’s Loving You” still raises goosebumps.
After watching Peter Weir’s Gallipoli I confidently predicted Hollywood stardom for young star Mark Lee. And what about co star Mel Gibson you say - no way.
Wow! Someone else saw that movie! 
Yes, that was probably her best role since “Hill Street Blues,” but most of her work are minor parts in forgettable films. A real shame.
Conchata Ferrell. She was terrific as April in the forgotten TV series “The Hot L Baltimore” and put in a very strong performance in “Heartland.” She was never going to be a star (she was a heavy woman), but her talent is badly wasted (she currently has a recurring role on “Two and a Half Men”).
Back in recording artists:
Emmitt Rhodes. His first album had him billed at “The American Paul McCartney” and it was filled with songs that easily would have been hit singles. His record company, though, didn’t promote him and he vanished.
I believe I first heard Jonathan Rhys-Meyers’s name being thrown around as a potential “next big thing” six or seven years ago, although I never saw any of his movies until about three years ago. A friend of mine had a crush on him and brought over her DVD of Velvet Goldmine one day. In his very first scene I turned to her and said, “This is the one you like?” “Yeah.” “Has he got a lousy agent or something? Why isn’t he a huge star? He’s got the face of a beautiful teenaged girl! Teenaged girls love guys who look like beautiful teenaged girls! There’s a fortune to be made here!”
The first Lord of the Rings movie came out not much later and proved my theory, but with Orlando Bloom as the subject. Rhys-Meyers has been working steadily for about a decade now, usually playing drug addicts, period piece deviants, or drug-addicted deviants in period pieces, and while I wouldn’t consider him a truly outstanding actor he’s certainly better than a few stars I could name. (Keanu, I’m looking at you!) None of his films prior to Bend it Like Beckham were particularly successful, though. Inexplicably, even this hasn’t won Rhys-Meyers mainstream fame or pin-up status. I think he’s better known in Ireland and the UK, but I don’t believe he’s really considered A-list even there. But he’s got two big movies due out by the end of the year (Vanity Fair and Alexander), and he’s supposedly been cast as Schmendrick the Magnificent in the upcoming live-action version of The Last Unicorn too. That’s got to be about as good as playing an elf as far as the teen girl set is concerned.
Rachel Ward, for all that she has 30 IMDB listings, hasn’t been the huge hit I expected her to be after The Thorn Birds and Dead Men Don’t Wear Plaid.
Mark Hamill had a surprisingly dead-end career, for all that he got to play the lead (sort of) in one of the higher-grossing trilogies of all time.
And if you’re going to start this off with Michelle Forbes, you might as well include a lot of the rest of the ST casts. Granted, most of them have gone on to produce/direct/write, but some of them are more than talented enough to land big roles–Brent Spiner, Rene Auberjonois, Andrew Robinson, Armin Shimerman… guess they can only land good roles if they’re aliens…
I second the Oded Fehr vote. He is a hottie…and can act. Naturally, Hollywood doesn’t want pretty boys that can act.
I agree that Michelle Forbes deserves more acclaim than she’s gotten (I always thought she has an Old Hollywood, Lauren Bacall-esque beauty to her), but as for some of those others, their careers are nothing to sneeze at. Brent Spiner has done some very high-profile supporting roles (and he’s not really a leading-man type anyway), and Andrew Robinson had some very noteworthy roles as a young actor in movies like Dirty Harry and Charley Varrick. Armin Shimerman had a regular gig on Buffy, and I see Auberjonois all over the place. Maybe you’re thinking of a different level of “success” than I am, but I would call all four actors uncommonly successful.
Keir Dullea (David and Lisa; 2001; The Fox). As Noel Coward famously said of him, “Keir Dullea, gone tomorrow.” The guy is a very good actor, adept with difficult and ambiguous characters, and was stunningly handsome in his 60’s heyday (and probably long after). But since the 1960’s, his career has been mainly limited to TV. 