Actually I've considered a mod like this before.
If you go with Jedi Knight: The Old Republic you can have the melee weapons be vibro blades(they have an electrical charge that makes it so they aren't cut in half by light sabers). Vibro blades could be spears as well. Also there are plenty of third world species that would use the equipment currently in the game.
The player could start out on as a normal person who then becomes a jedi. The time set could be when the Jedi were one and then the civil war broke out. That would explain the large amount of jedi fighting jedi and lightsabers. This would allow the player to choose from two sides to fight on creating replay value. This would also explain why you don't leave the planet.
Lightsabers could theoretically be two handed weapons, single handed weapons, staffs(double bladed lightsaber) and pole arms. Different lightsaber statistics would be explained by the different crystals used to create them.
As for blocking missile weapons with a lightsaber- someone already tried replacing a shield graphic with a weapon so it appears they are using two weapons at the same time. It worked and the weapon in place of the shield worked exactly as a shield does. So you could provide "special lightsabers" that are really shields with a lightsaber graphic. These "special lightsabers" would be explained as the only lightsabers with the crystal that allows them to block blaster shots.
Creating blasters would follow the same method that they are using to create the guns in the Wild West mod. Throwing weapons would simply have the blade replaced with a laser blade and the haft could be made to look metal with buttons.
You would have a problem explaining horses, but once we can change horses you could probably replace their graphic with speeders, Star Wars creatures, hover craft and the like.
As for cities someone started a Tron mod and I don't know how far they got, but I saw they they radically changed the look of a city.