I don’t even know if any bands still do this. A guitar solo at the end of a song that keeps going as the song fades out, no actual end to the song. When I think of this two Steely Dan songs immediately come to mind for me. In both cases I wish I could have heard what was going on after the music faded.
Kid Charlemagne: Steely Dan. Guitar solo by Larry Carleton
My Old School: Steely Dan. Guitar solo by Jeff “Skunk” Baxter
I think it will be hard to top those but give it a shot.
God, this might sound cheesy and will definitely date me, but the first song that came up when I read the thread title was Running With the Night by Lionel Richie. I don’t know who did the guitar work on that song, but to my 16 yo ears, they had far better chops than one would expect from, you know, Lionel Richie music.
The ones I thought of were “Rainmaker” by Traffic and "The Battle of Glass Tears"by King Crimson (although the guitar has settled down somewhat by the end of the latter).
On Ryuichi Sakamoto’s album The Thousand Knives of… the track The End of Asia features an amazing guitar solo by Kazumi Watanabe. It’s not exactly a fade-out, as the song cross-fades into another song, but the guitar doesn’t show up until 4:30.
It starts at 5:37. It’s just a beautiful ending to the record (I don’t count “They’re Red Hot,” since I think it’s basically a throwaway track) and so different to most of what’s come from the previous 60+ minutes.