I know this thread is very old, but if anybody's interested. I've started working out an XCOM rpg using CortexPlus, the system that Leverage RPG uses. It's very flexible and robust so I'm excited to see what it can do with XCOM.
I've only played UFO: Enemy Unknown (and never even got halfway through it), so that's where I'll focus. Check out my thoughts on the design process and let me leave comments with your thoughts.
What are the most important/fun parts of UFO Enemy Unknown that you want to see in an RPG? If you have done any work on XCOM rpgs I'd love for you to send me links to any info or number crunching you have. I'm not a fan of number crunching myself so I'd greatly appreciate benefiting from your expertise with the game.
Check it out, and follow along as I explore the process of crafting an rpg worthy to be associated with XCom on my blog Exploring Infinity