Looking at Scoundrel, one of the things I found unusual was the addition of the Mace trait to some weapons. Weapon traits had not previously been a thing, to my knowledge (I'm sure the Central Supply Catalogue is absolutely full of 'em, but that's not a source we have). In any case, it got me thinking, so here're some traits and weapons to go along with them. A bunch of these started during an abortive attempt to convert Dark Heresy to MongTrav - I really like DH's premise (group of dudes working for an Inquisitor hunting heretics) and setting (40k), but their mechanics are a bit baroque. Trav's mechanics, on the other hand, are nicely slim, but the setting / premise tend to be relatively uniform; group of dudes out for profit in a space westerny setting. Anyway, got bogged down in the career conversions, but weapon traits are easy.
Finesse - A Finesse weapon is best used to accurate strikes against important body parts. In the hands of a skilled user, it is exceptionally deadly despite likely having lower base damage than other, similar weapons. When using a finesse weapon, double the effect of the attack roll for determining the bonus to damage from effect (ex: If you roll a 10 total, that's effect 2. With a normal weapon, this gives +2 to damage. With a finesse weapon, this provides instead +4 to damage). This trait would be applicable to precision melee weapons like stilettos, but was originally an adaptation of DH's Accurate trait, which provided bonus damage on sniper rifles for hits with high effect.
Stiletto - TL2, Range: Melee Small Blade. Damage 1d6. Heft -, Mass -. Cost 30 Cr. Finesse.
Sniper Rifle - TL7, Range: Rifle. Damage 3d6, Auto -, Recoil 1. Mass 5kg, Magazine 5. Cost 500 Cr, Ammo 10 Cr, Finesse.
Reliable - Reliable weapons are reliably destructive. When you hit with a reliable weapon, roll damage twice and take the better. In a D&D conversion, this trait might be used for weapons that roll 2dn rather than 1d(2n), such as the greatsword, which have higher means and lower variances than other similar weapons such as greataxes.
Double-barreled shotgun, Mk2 - TL4, Range: Shotgun. Damage 4d6, Auto -, Recoil 2. Mass 3kg, Magazine 1. Cost 100 Cr, Ammo 3 Cr, Reliable.
Explosive - An explosive firearm fires bullets containing small amount of explosives, which detonate on impact. While these weapons are banned under the Geneva Convention for use against human targets, there are no such universal restrictions in Trav-land... When rolling damage with an explosive weapon, you initially roll damage as normal. For each 6 that comes up on a damage die, roll another d6 and add it to the total damage dealt. Repeat this for any 6s rolled on the additional dice. This trait could also be used to model weapons that occasionally spectacularly obliterate targets (like for modeling x3 or x4 crit weapons from D&D). Another interesting tweak would be to vary the numbers on which dice explode; maybe for an x3 crit, they explode on 6, but for an x4 crit, they explode on 5 or 6.
Bolter - TL11, Range: Assault Weapon. Damage 4d6, Auto 4, Recoil 1. Mass 5kg, Magazine 12. Cost 1000 Cr, Ammo 50 Cr, Explosive.
Bolt Pistol - TL12, Range: Pistol. Damage 4d6, Auto -, Recoil 2. Mass 2kg, Magazine 6. Cost 700 Cr, Ammo 25 Cr, Explosive.
Hazardous - These weapons are dangerous to the user's health. On a miss with effect -2 or lower, the wielder has managed to injure himself, inflicting a hit (unmodified by effect, but modified by armor) on himself. As these weapons tend to be unusually damaging, this is often a bad thing.
Chainsaw - TL7, Range: Large Blade. Damage 5d6, Heft 3, Mass 6kg. Cost 350 Cr, Hazardous, Explosive.
Lightsaber - TL14, Range: Large Blade. Damage 4d6, Heft -1, Mass 1 kg. Cost 15000 Cr and membership of one or more psion orders, Hazardous, Ignores Armor.
I have some more trait ideas, but this serves as a first attempt into the field.