I also have a few ideas considering the topic:
One moonphase equals one season (so 8 in-game days)
Changes:
Only affect tiles above ground
Affected Areas:
Forest Biome
Evil Biomes
The Hallow
Biomes that are not - or just partly affected by season changes are:
Jungle
Tundra
Desert
Ocean
Mushroom biome
Spring:
The usual textures: so spring is the default
No real changes
Summer:
Forest grass turns to lime green
Evil grass, Hallow grass stays the same
Tree textures stay the same
Maybe a heatstroke feature if you're near campfires during the day -> Negates the healing effect / heatstroke feature during the day when in desert or jungle biomes
Increased enemy spawn rate -> +20%
High chance of thunderstorms -> +20%
Herbs grow +20% faster
The nights are a bit brighter than normal
Autumn:
Forest grass turns to orange
Evil grass, Hallow grass stays the same
Tree textures change to a autums style (orange leafes etc.)
Increased enemy spawn rate -> +10%
Low chance of thunderstorms -> -20%
Higher chance of rainy weather -> +20%
The nights are darker (like in the Constant seed)
The Halloween event naturally starts (So you're not dependent on the real life date or beating pumpkin moon)
Winter:
Forest grass gets a new texture (a bit snow)
Evil grass, Hallow grass also gets the new snow texture
Tree textures change to a winter style (with snow on the top, or no leafes at all)
Decreased enemy spawn rate -> -10%
No Thunderstorms
Blizzards replace Rain
The nights are darker (like in the Constant seed)
The Christmas event naturally starts (So you're not dependent on the real life date or beating frost moon)
Ice slimes replace regular slimes
Eskimo Zombies and Tundra critters start spawning in the Forest
Being outdoors while a blizzard gives the chilled debuff (gives the handwarmer finally a true use) / chilled debuff wehen in Tundra biome at all times
Deerclops can naturally spawn in the forrest now if the conditions are met
i think those chances could bring a fresh new feeling to vanilla terraria.
Maybe considering these changes being part of a secret seed combination with the constant seed.
I really hope the devs read this and like the idea haha
Btw. I'm new here, thank you for the attention