In Team Fortress 2, there's various parts of a system for creating items with different attributes, similar to what I think you're suggesting. Theirs is in two parts: various "unique" weapons that come pre-determined with different names, and ones with simple stat adjustments. I'm not sure whether you're looking for one, the other or both.
For instance, you could run into a person who's carrying the unique 'Force-A-Nature' shotgun variant. No matter which person has it, it always has these attributes:
Blockhead is wielding The Force-A-Nature
Knockback on target and shooter
+50% faster firing speed
+20% bullets per shot
-10% damage done
-60% clip size
The other method has various generated items, where one of the default types in the game is modified with some positive and some negative stats and given to one specific player's loadout. For example, a person finds or is given this:
Blockhead is wielding Common Pistol
+25% damage done
-30% fire rate
In the actual game this wasn't used much, only to test it by giving developers ones with lots of overpowered functions for fun, but in Blockland this goes very well with the event system so you can add functions to specific player's items e.g. "bow shoots fire arrows" for a challenge, "explosive shotgun" powerup.
For the 'unique' items I'd suggest having them all created from a file when you create the server, with the changes listed:
config/server/statWeapons/mortar.txtMortar L.
BASE Rocket L.
HIDDEN PROJECTILE gravityRocketProjectile
DESC "Rockets launch in an arc"
DAMAGE 1.2
RADIUS 1.5
SPEED 0.75
RATE 0.67Mortar L.
Projectiles launch in an arc
+20% damage
+50% splash radius
-25% projectile speed
-33% fire rate
For the other system there'd be some events where you pick a weapon type (e.g. any, all Bows, specific "Iron Sword", current slot) to either apply an individual change to plus a value or to clear changes for.
Note that if this was to be made, in order to adjust the fire rate it would have to apply only to weapon types specifically made for it and changing the projectile to ones with special effects would most likely not work with changing the damage amount.