Here's one that just occurred to me: THE UNTOUCHABLES. The 1987 de Palma and Mamet film. It opened on June 3 of that year, making it just over 20 years old. And if it were coming out now, slash fandom would *eat that movie alive.*
It's a great cast: Kevin Costner (Ness, the Treasury Agent), Sean Connery (Malone, the old beat cop), Andy Garcia (Stone, the young sharpshooter), Charles Martin Smith (Wallace, the FBI accountant), and Robert de Niro (Al friggin' Capone). Fandom OTP: Ness/Malone. Because the in-film relationship between the two is so strong, that's the obvious slash pairing. Also, look at Connery: he's like a bull in that movie; his fists are the size of hams. Young Costner is as slim as a greyhound. Slash fans love that pairing of two different physical types, so they'd be all over that. The confrontation between Garcia's Stone and Connery's Malone, when Stone is recruited, is really well-played, so that'd be my bet for the secondary pairing. Though there would be a fair amount of Capone/Ness. Actually, knowing slash, there would probably be Capone/Ness rapefic, followed by Malone/Ness hurt/comfort. Oh, and Wallace goes nuts in that raid on the shipment (with the memorable mounted attack followed by a shootout on the bridge) when he thinks Stone's been shot, so my bet is that Wallace/Stone would be another popular pairing. Also, except for Ness, none of the men are married, and there are maybe two speaking parts for women in the flick. This simplifies things for those slashers who see women as obstacles to be overcome. (I would expect a lot of "Capone kills Ness's wife and Malone helps Ness to move on" fic, though.)
It's a little weird to find myself thinking in these terms. On the one hand, it's amusing to imagine; on the other -- wow, it's kind of cynical in parts, isn't it?
What other films do you think "missed the boat" in similar fashion, but would have had a big slash fandom if they were opening today?