I'd add something to the Snow Biome, as I feel the atmosphere is the best, but it lacks some kind of incredible enemy that really sets it apart from other biomes. I'd go for something badass haha, with some really classy drops.
Something like this:
-
Frost Zealot
- Found in a small, broken throne room in the Snow Biome.
- Can be summoned by finding 3 throne pieces throughout the Snow Biome (only 3 can be in a world at a time). He'd be a hardmode boss (Golem-tier), but wouldn't affect the progression of the story.
- Resembles a Paladin, but with a more humanoid physique, and is much bigger. Also has notably icy eyes, and a dull silver armor re-skin.
- His behavior would involve a first 'conservative' phase where he swings his sword once, then blocks with his shield (making any attack aimed at his shield void). At 1/4 of his health he would explode into some kind of Ice Demon, and increase his attacks, sacrificing defense.
- Potential drops
1. Fanatic's Blade (Sword) 20%
2. Blizzard Helm (Helmet) 20%
3. Crusader's Musket (Gun) 20%
4. Greaves of Cruelty (Boots) 20%
5. Cloak of Doom (Vanity Cape) 20%
6. 20 Gold 100%
7. Couple of Potions 100%
8. Unstable Snowball (Summons a mini Cultist. Gotta add to the collection, right people

) 1%
Let me know how crazy an idea you think this is haha.