I actually would really like to see low-grade fuel for something like a flamethrower. I think some type of futuristic laser weapon might be okay in Rust, but it would have to have the right aesthetics (dingy, beatup kind of look) and more of a solid beam that takes batteries you could charge using your generator kind of thing. Maybe have it with bonuses against certain armour/buildings and penalties vs. others.
What I had more in-mind would be more of using an industrial strength laser to cut through doors, like a type of siege weapon. It would take power plugged into it, or charged batteries, and would be vulnerable to attack, but could be one of the best ways to cut through metal walls, could use it to kill players at well, but wouldn’t really be as effective at that.