Where there is one guitarist throughout the album who is the main focus of an album, but has pretty much each song done by a different vocalist? Or, an album full of different guitarists playing different styles all strongly centered around a very strong main theme (like Merry Axemas is very strongly related to Christmas).

there's Strait Up, the last Snot album. it's all pretty nu metal, though

Most of Ayreon's albums work that way, to some degree (there are often multiple singers per song, and depending on the characters in the story, some singers show up multiple times).
Primus' Antipop is basically the reverse - different guitarist for every song.
Jeff Loomis' Plains of Oblivion is mostly instrumental (with guest guitarists), but the 4 songs with vocals are by a different vocalist each.

Carlos Santana's "Guitar Heaven: The Greatest Guitar Classics of All Time" is a cover album with a lot of guest artist doing vocals to some of the most memorable classic rock songs of all time, really fun to listen.

There's Dave Grohl's Probot, where there is a different singer on each song.