yo! scratchbuild fantasy weapons are hard to make because they arent someone else's ideas, but they turn out wicked cool. a few tips to keep in mind.
lasers are a big part of futuristic weapons, whether it be targeting or the actual gun, they are a nice touch.
light up components also add a nice touch...
in construction of detail, keep in mind what kind of futuristic weapon it is designed after, so you can refer to that weapon for pics. bondo is ggod for adding detail.
a good back story is a great idea when creating a prop. whats its clip capacity, what color is it, who first designed it, are all key elements in a back story.
again, the only limitations is your emagination. :lol have fun with it!