An idea I've entertained for a long time (or rather, it entertained me) is mixing element of fantasy and science fiction, as well as some other stuff. Granted, I've been in a low-magic kind of mindset lately, but here's my idea of a more fantastic campaign.
High technology would allow castles to defy gravity and float in the sky. Maybe the same technology (or magic if you prefer; it's still fantasy) will have sailing ships that fly in the sky to reach those castles. Knights will wield light sabers (maybe even light lances) and ride robotic horses.
Since it's still fantasy, fantastic elements will still be around. There would be dragons, elves, and orcs. Martial artists can fly around wuxia style and shoot energy blasts at their enemies, a la Streetfighter 2 or DBZ.
The main problem would be weapons. Since light sabers exist, other melee weapons using energy should exist. Maybe maces can produce sonic blasts. Axe blades may vibrate. As for ranged weapons, how about crossbows like the one Chewbacca used? And the bows like Hank the ranger in the D&D cartoon used would work too.
Armor would probably be best handled as force fields. Maybe damage reduction would work too.
Suggestions?