Have you thought about just adding a hook to something like total economy for your “goldcoins” aspect of drops?
All in all I like the idea dude! I will be lurking.
I used to use MoneyDrop on my bukkit server. Having monsters drop currency is vital for any RPG server but I have always been a lover of simplicity, instead of creating a new item through Forge maybe just make it have the appearance of a gold nugget? The less I have to put into my Forge package for players to upload the better! 
It should be on your own what kind of “visualisation” your currency get
But like you already said - I’m also a friend of simplicity - So no hooks.
have you looked at MythicDrops and RPGPlayerLeveling for spigot i love them plugins and would love it if you could make somthing like that for sponge
I’m already on it 