Rihanna would flop with 'Born This Way'/some of 'The Fame Monster' material, maybe because she still isn't seen as talented/credible enough to pull off something edgier (see the whole 'Rated R' which was saved by 'Rude Boy', but even something a little less generic like 'Man Down'), and honestly I don't see her voice giving that
quid to songs like 'The Edge Of Glory', 'Paparazzi' or 'Bad Romance', especially considering big part of the latest two's success was due to extremely 'Gaga' videos... the only Gaga singles I could see smashing if sung by her are 'LoveGame', 'Just Dance' and 'Telephone'... maybe 'Alejandro'. I won't even get into the thought of Gaga singing a big part of Rihanna's discography, but generally I don't think it fits her persona/voice.