I always liked the concept of the evil biomes spreading.
The times i played on singleplayer worlds,when hardmode activates, i always feel that im on a hurry, that something must to be done to stop the spreading quick, feeling an increasing sence of pressure , and keeping on mind that the time is valuable,and the more time i take, the faster time depletes.
Usually this tecnically doesn’t affects progression,and shouldn’t really have much preocupation.
Giving more focus on the spread is something i would like a lot, and at some point “Forcing to care about the spread” without being very annoying,
Things like starting to see Crimson/Corrupted enemies on pure biomes if the spreading is high enough, Starting to see decayed trees everywhere, an idea of contaminated mineral ores making needing more to craft an ingot, Music being a bit altered, etc…
Mostly the idea, is to make notice the player that the world is dying,That even if the evil spread haven’t reached that area, It has consecuences, and something must be done.
Alongside this, aswell a better (and less boring than making hellevators and spamming clentaminator) way to purify the world, that rather thanbeing the spread “Equal” make some areas more “Affected” like seing more enemies, or other variant of corrupted block, as a hint that the “Core” of the problem is near
And about cores, Adding a “Core” in this areas, that after destroying it, inmediatly purifies that little zone near, Like a 100 tiles radius or an irregular area/chunk of blocks/“Sectors created by the game”.
Clentaminator would be still avalible, but this may be seem more interesting to people that are more about exploring and fighting, (Or mining as my case).
And at last, i think i have seen some concepts on Duke fishron, but im not entirely sure
That the bosses change according to the world, i.e. Not being the same fight an “Duke fishron” on a pure world than in a corrupted one, Changing his design(sprite) and adding 1 or 2 new moves to make it a bit more harder.
Overall, the general idea is giving the spotlight to the evil biomes spreading a bit more (Sort of what Terraria otherworld showed in the traillers).