They could do great numbers with "Secret" and "Don't Tell Me." And, of course, "Express Yourself"! Perfect given how many of the characters have come to self-acceptance through participation in the glee club. I'm seconding you on "Like a Prayer" (sing it, Mercedes!) and "Take a Bow," which is one of my favorites. Personally, I'm a big fan of the message, insofar as there is one, in "What It Feels Like For A Girl." While I don't think it would work as well for the show, I'll definitely be doing a little happy dance if they do use it.
"Borderline" -- my all-time favorite Madonna song
All of the songs have actually been released if you're interested in knowing exactly what will be in it! :)