This is fantastic. I'm surprised I haven't seen something like this before. I think you've got a great potential market here for monetizing. It's like stock photography meets product upsell.
My first suggestion would be to get similar shots to those you've already done with a female replacing the male. It's an easy way to add more shots quickly.
I also like the idea of more scenes including people. Make it look like they're having fun and also using the phone/tablet at the same time. Smiling faces sell products!
"Hands holding photoshop-able screens in environments" is rich photographic territory. I would also love to see fat hairy hands, dark-skinned hands, waxy old hands, prosthetic hands, etc.
My first suggestion would be to get similar shots to those you've already done with a female replacing the male. It's an easy way to add more shots quickly.
I also like the idea of more scenes including people. Make it look like they're having fun and also using the phone/tablet at the same time. Smiling faces sell products!