Great idea! In terms of balancing I'd think about an item, much similar to viking helmet or wolf pelt. So you need to sacrifice a precious item slot for it.
So maybe a pirate hat or something that lets you count as darkness, or a sparkling top hat to count as metropolis. This way you'd need to sacrifice an inventory slot.
For nature we'd need a buff tower first. Time to finally add Yggdrasil, the world tree :-)
Currently I'm wondering what would happen if you put all three items on. Probably count as three elements? I think it shouldn't mess with your quests, though...
Haha, Imagine having a tower that's a sailing boat (on water if it works).
Equipping the Pirate Hat on the boat. Making those two a set for great bonuses!
Set name "A Pirate's Life For Me"
Set bonus
"A %chance when getting kills, to make the enemy a pirate crew instead, adding %gold/luck to all metropolis towers"
(Similar to abyss)
And with the sparkling top hat equipped on another nature, making it a metropolis tower to gain extra gold/luck.
Like stacking shadows adaption faster or giving it to the Abyss Knight for more luck to increase his army... or other tactics you come up with.
Also with the pirate hat, the boat becomes darkness, so it doesn't gain its own effect. Unless you also equip the sparkling top hat aswell maybe.
Maybe it could gain its own bonuses + Abyss knight bonuses. Becoming metropolis and darkness?
Endless ideas...
Sorry for wall of text, I hope you atleast read it Andy! Haha