I actually don't think this movie would make a good musical at all. First of all, I think a road trip movie is really hard to transfer to the stage. Also, I really am against the recent trends to transfer popular recent films to the Broadway stage, because I think the film performances and story are so freshly kept in the audiences' minds than anything that is on stage comes off as an imitation (see THE WEDDING SINGER, LEGALLY BLONDE, SHREK, etc). Toni Collette, Alan Arkin, Greg Kinnear, Paul Dano, Abigail Breslin, and Steve Carrell all did a brilliant job in creating these roles. Yes, I know the stage is a different monster, but I just don't see why anyone should tackle those parts so recently.
I guess I'll have to wait and see, but unless this adaptation is completely different from the film (in the same way that say A LITTLE NIGHT MUSIC and NINE are different from their film counterparts), I don't think there's a reason for the show to happen.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"