Another Idea! This one is a biome which would have conifers, treants, thieves, and thief outposts. A thief outpost would be a smallish bunker which is half above and half below ground. It would have banners and spawn thieves. Thieves would take money from you on hits, like a reverse Lucky Coin, and would have a chance to dodge attacks. There would be many varieties of thief, like Skeletons, and there would be Gunsmen with muskets. In the outposts there would be a chest with loot like money, throwing knives, brass knuckles (an accessory which would allow attack with anything you hold by right clicking. Or this could become a new form of weaponry, with it's own button), and thief armor, which would provide 2-5 def per piece, with movement and attack bonuses. The set bonus would be a swipe attack; by pressing left or right before you land on an enemy (presuming you have started to jump on the enemy) you would deal a fairly large amount of damage for Early game, at the cost of some of your own HP
Treants would be a form of Mimic which would go behind back walls if any, meaning they cannot attack you but you cannot attack them. They would look like like a kind of everscream but smaller, They would also pelt Pinecones at you.
This biome would activate after the EoC has been defeated, saying "The forest is becoming unruly..."
Overview: a more forest-like biome, as the normal forest feels rather stripped of trees. More like grasslands than forest. However, this being 2D you cannot really overlapp trees.
I cant think of a name right now for this biome, nor can I think of any more ideas for this current time.
Please feedback in comments. Did I do alright?