I don't want to spoil anything, but we played post-apocalypse with d20 modern and it was boring. I really loved d20 modern, but fire fights arethe obvious choice once you have aquired fire weapons. It' just much mor effective. And fire fights in d20 modern and static and all but interesting or tactically challenging. I guess ranged combat just isn't the d20 thing. So maybe you wanna think about some improvements to that area of rules.