There's a third party supplement I use for upgrading weapons called Ancestral Weapons. Basically, you get predetermined amounts of points to spend on upgrades that make the weapon better at certain things or entire certain abilities. As you level up the weapon by hitting predetermined milestones established by the player and DM (i.e. slay a high ranking follower of Asmodeus with Grandpa Paladin's gun), you gain more points and the levels of upgrades you can unlock get higher. It's fairly elegant overall and lets you shape the weapon as you go, rather than having to decide what everything can do up front.