Vendors can offer any mk0 weapon. As you find/unlock weapon upgrades/attachments /blueprints they are also unlocked in the vendor. 3-4 random available with a credit sink to re-roll available upgrades prior to the run. Or even has the full list you've unlocked to select from to encourage build craft (this is the one I'm leaning towards personally)
During missions you can find weapon upgrades/attachments/blueprints that you can apply to your weapon. These can either be agnostic to the weapon type (i.e. damage, magazine increase, etc.) or be weapon specific (i.e. x4 scope for the javelin or slugs for the maw). If you finish a mission after discovering them then you unlock them to go into the vendor prep menu. You can keep the weapon you have equipped as long as you like, until you fail a mission, and can freely swap upgrades with mods found in mission or can change upgrades from the vendor between missions for a cost.
I personally believe this would lead to more investment in the weapons and give a sense of progression instead of just hoarding mk3s and not caring about loot past that. This suggestion is made fully from my point of view that weapon carry over is not a good long term mechanic and that small arms should not be part of the inventory slots as they are currently. Multi-tool, chosen small arm, and melee of choice +3 inventory slots for actual loot.
This would be slow incremental changes over a long time. They are small team and other things require priority. This system is an end goal to work towards. Not something to expect in the short term.
