From an interview with Empire. The article speculates the project may be the previously-announced "Musical epic" "X-crucior". But even that announcement had no specific details aside from the script and music being by Ron Mael and Russell Mael (aka Sparks).
Another interesting tidbit from the article - John Woo was one of the directors who tried to bring Phantom to the screen prior to Joel Schumacher getting the gig, and Woo's pitch would have had his Broken Arrow and Face/Off star John Travola in the titular role.