What would you think of a Legacy weapon system or something similar in only war? Some guardsmen de get attached to their weapons (see Psycho demeanor) and perhaps it may be a good idea for a guardsman to show that "this is my boomstick"? I am aware f the weapon customization, but as of right now it is very limited compared to the BC system and only applies to standard kit weapons.

I'm on the fence about it, because I'm not sure how it would affect the game on the mechanical side.

it COULD work, but it would require some retooling, both fluff and mechanics wise. I would also be very careful in deciding what can and cannot be a legacy weapon. standard kit? spoils of war? sure. requesitioned items? only if they're on long term lease, and you want to roleplay a scenario where the characters have to explain to the Munitorum why they're not getting their (now probably illegally modified) multimelta back.