No matter how futuristic the weapon, you should always get inspiration from real weapons. The way they are made, the way they move, all the details that real weapons have can be applied to futuristic weapons.
Imagine yourself holding the weapon, how comfortable would it be, where would you want hand grips and reloading parts.
I think you should try and make a modern version of a traditional weapon, try adding as much detail but with a modern design. Even making a traditional weapon is good practice, you learn how to add those details and where to spend polygons where it counts - if it's for first person then the top and left side need more detail. Texturing a real weapon is so much easier as well. Practice makes perfect, you have the basic skills so why not try for something more complex.