GURPS, well it has issues. It does a low-powered mundane sort of game well within limits. But it breaks and breaks fast as the power scales upward. By the time D&D PCs reach 10th level they've essentially blown the top off the GURPS power scale. Just the racial traits of Demons alone come in at a bit over 700 points.
Risus or Spirit of the Century are good at what they do but they take cinematic play to nosebleed inducing heights.
So I'd recommend sticking to d20, it's the most flexible easily modified system I've ever used. You can tweak into entirely different genres with a few house rules. I've done everything from prehistoric caveman games to mecha/powered armor/and starships. I've had space rangers fighting classic D&D heroes to recover a lost Panacea Machine being used as a Holy Relic.
Right now I've got a planescape game running where the PCs consist of
1.) Downed fighter pilot who came through a portal, plays a cleric (searching for his god on the planes) dressed in chainmail over his old uniform wielding a shield and a holy Thompson submachinegun.
2.) Orc barbarian/wizard from a primitive stone-age tribe who worships Odin, wears hide armor and carries a giant's greatclub.
3.) Archetypal D&D sword+board fighter except he's an anthropomorphic dinoneichus(sp?) who follows a variant of the Bushido code and views kobolds as holy emissaries of the Heavenly Dragons
4.) Autonomous Exploration Robot from a star empire that looks sort of like a cross between a centaur and a crayfish. With heavy armor plating, big metal claws and a turreted laser carbine. Uses a 3rd party technologist class.
5.) Very Young Rust Dragon, no class advancement yet. But wears snazzy bronzewood claw extenders and tailmace.