[jwyss234]

Neca has made a few diorama pieces but you'd probably have to search eBay for them. I think one was a trophy wall but you'd need to Google it to find out what all they have made. They also put out a Predator accessory pack last year so you should have any easier time getting that. Neca provides downloads for back drops at the end of each year.
Diorama Downloads | NECAOnline.com

I can't think of any diorama set that you could just buy so you may need to make it yourself. You could experiment on your own or you could search the internet to see what other people are doing. I don't have any specific resources to point you toward for this.

For characters that the Predator can fight, Xenomorph Aliens seems like the natural choice. Probably any Neca figure could work like Ripley, Dutch, DC movie figures, Terminator, Robocop, Ninja Turtles, etc. If you missed out on Dutch a few years ago, he'll be getting rereleased later this year. Neca is also releasing some DC vs Alien/Predator sets later this year. The first set is a comic book Batman vs a Joker inspired Xenomorph. Outside of other Neca figures, you could try Star Wars Black Series. You'd have a wide selection of alien bounty hunters and troopers as well as iconic characters like Darth Vader. You could also try Marvel Legends figures like Wolverine, Punisher, Hulk, and Spiderman.