I always wanted to roleplay Sid Meier's Alpha Centauri. GURPS even came out with a supplement for it (following the very nice formula of recycled art = lower production cost). Of course, the questions raised by the video game -- the transhumanism, ecoextremism, social commentary, the basic "how shall we live?" and so on -- are difficult if not impossible to raise using GURPS as a chassis. The GURPS book will allow you to roleplay in the world of Alpha Centauri, but it doesn't support playing Alpha Centauri. In precise terminology, it does not encourage the address of the question "how shall we live?"
That'd be neat to play a game that answered that. Shock: comes close, but it's more "How will this affect our lives?" and has less to do with making personal life choices.