Applause was the only real choice for lead single, let's be real. Do What U Want and Gypsy could have worked as well. I'd have been interested to see how Aura would have turned out but I don't think it would have done that well.
I think it relayed too much on the MV impact. The plan was releasing the song + video the same day so when people would have find out it was the Telephone sequel it would have won the fans. Also, it has that wtf moment so I don't think I would have flopped.
I wouldn't mind if Trashion! was a single, it has mass appeal and accessible lyrics, also a Will.I.Am and David production, if that doesn't scream radio friendly then idk what does.
Fashion!, Gypsy, and Sexxx Dreams are the only songs with single potential at this point. Excluding G.U.Y. but I think she is ending the album with it.