We Found Love - Cheryl Cole

How We Do It
Broken Hearted
Young Forever
Domino
Spectrum

Katy could pull them all off effortlessly

Woulda been epic tbh.

"With Love" for Britney Spears
"Me & U" / "Long Way 2 Go" for Rihanna

i cannot see Rihanna on Say It Right, that is all Nelly Furtado. I could totally see Katy Perry doing Promiscuous.

YES.

Katy Perry singing Pink's "Heartbreak Down"
Avril Lavigne singing Kelly Clarkson's "Already Gone"

This isn't exactly what the OP is asking but I feel like Nicki wrote Lil Wayne's part of Roman Reloaded and I can see her rapping it too. "Scuse my french two bitches and moi, that's a Nicki Minaj a trois"

My flop thread only 2 dumb pages

Call Me Maybe by Taylor Swift.

This thread flopped