I would have loved to hear Glenn sing "Life In The Fast Lane". I think he did an incredible job on "Desperado" on Letterman when he sang it. I like it almost as much as Don's version.
Glenn could rock a song - look at HT, AG and OOC. I never understood why Glenn never sang LITF and some of the other songs on HC & TLR. He certainly could easily have handled them.
"The Henley Takeover" side is interesting. I think in the beginning Glenn thought it was better for Don to sing lead but later, based on some interviews he did back in the 80's, he indicated he had given up too much and would never do something like that again.
IMHO - and just my opinion, I think the Eagles suffered when they went from multiple lead singers to just Don. I love his voice but, I think they missed something when it just became all about Don. Glenn, Randy, Joe and Tim all should have had more than one song on an album. I think this is one of the major reason why so many people think of Don as the only one who could sing and dismiss Glenn as the "least talented Eagle" to quote people on you tube.