I don't know if you've looked at d20 Modern, but it seems like a pretty good system for an XCOM RPG. You could simply make an XCOM sourcebook with all the weapons, skills, feats, etc. appropriate to XCOM. d20 Modern has classes, but they are based on the basic six d20 abilities (Strength, Dex, etc.) and it seems to encourage cross-training. It still has levels and D&D-style hitpoints though. Perhaps d20 XCOM could do away with the hitpoint gains tied to level gains; maybe the only way to gain hitpoints would be to increase Constitution. Good luck with the project. When/If I see it at my local games store, I'll definitely pick it up. Like someone else said, I'd buy it for the UFOpaedia alone.