>Almost every time I pass the Hard Rock Cafe, I think about what a shame it is that that isn't a Broadway house (was it ever? movie house? vaudeville?)<
It was the Paramount Theatre, a 3,600 seat movie theatre/concert hall. It was gutted years ago, with the space being used for office space and for the New York Times next door.
But as far as Broadway theatres that are serving other purposes, the two biggies are the Mark Hellinger and the Hudson. The Hellinger, on West 51st Street across the street from the Gershwin, is currently the Times Square Church, but it is one of the best Broadway theatres for big Broadway musicals. It was sold outright to the Times Square Church in 1989, and who knows if we'll see it back again anytime soon. It seated around 1,600 and is an incredibly beautiful theatre.
The Hudson is on West 44th Street, near the Belasco. It is part of the Millennium Hotel and used for events. I think it has limited wing or fly space, and was mostly used for plays, but it's a gorgeous house. It was recently restored, with beautiful Tiffany mosaic glass work uncovered. No idea if it could feasibly house a Broadway show again, but I'd love to see it happen.
In the category of pipe dream, The Ed Sullivan Theatre, home of the David Letterman Show, was briefly the Hammerstein Theatre when it originally opened. It only served as a Broadway house for maybe 8 years at most, but has spent most of its existence as a TV studio. I'm curious what its fate will be whenever Letterman hangs up his hat.
Begin at the beginning and go on till you come to the end: then stop.