Falko had a good idea with this. For example, to get either laser or gauss as a early gun:
- Advanced Weaponry research project: getOneFree: Laser Weapons OR Gauss Weapons
-> Player randomly gets one but not the other, representing the scientists making a breakthrough in either one of the fields instead of both at the same time.
If you want to make both eventually available, you can make a "Laser Weapons Gauss unlock" which gives Laser Weapons and has dependencies on having developed the whole Gauss tree, representing the scientists getting another idea for weapons tech after perfecting the first breakthrough.
It's actually quite nice and an interesting way to mix up how the game progresses. You could even make it so that "Advanced Weaponry" unlocks either the pistol or the heavy weapon, making it so you either develop the high power system which needs to be big, or the miniaturized system because you haven't yet figured out a big power source.