My first answer would be either
HERO System or
GURPS, since it's eay to set up packages or templates...
I even tried to create an MP variant using the
Millennium's End system from Chameleon Eclectic. (How I miss that game!)
Now my current thinking is, if one was to do it with d20, use the rules and classes from
Stargate SG1.
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Now playing:
Nightwish - Wishmaster
via
FoxyTunes