I feel as though the other roles are easy to cast as stars. I picture it in the round with a sandbox al la OOTI but the stage is at least 3x as large with 3 large play areas where scenery can change off camera.
"People have their opinions and that doesn't mean that their opinions are wrong or right. I just take it with a grain of salt because opinions are like as*holes, everyone has one".
-Felicia Finley-
That would be the biggest "Why?" question if this theoretical event should happen. What is there really in the role of the Narrator to attract Gaga at this current point in her career? It's not like it's a huge acting challenge. It's not a role that would help springboard her into a new direction (unless she's really looking to leap into bubblegum fluff at this point).
And staging a live event in the round would just be odd, unless you don't mind seeing a crap-ton of camera equipment and crew in the background of a majority of your shots.
These live broadcasts aren't meant to advance the careers of their stars. The star casting is meant only as a way to attract an audience. If Lady Gaga is a fan of Joseph, she'd do it because she's a fan. Carrie Underwood had no career reason to do The Sound of Music, None of the cast of the Wiz needed career reasons, Hairspray had the right caliber of talent, but wrong caliber of fame, but they didn't need it for career advancement. Same with Grease and the recent Jesus Christ Superstar. These stars did the broadcasts because they liked the material, and considered it to be a fun project to work on (and a challenge since it was live). Who knows why anyone did Peter Pan; they obviously couldn't get anyone with any real drawing power to do it, but Allison Williams described how much she loved the role and did it for that reason.
Ever since A Star is Born was released, people seem to be treating Lady Gaga as if she were some new discovery. She's not.