I looked forward for that
Some Ideas:
Purify X Blocks of corrution (doesn´t matter how)
Corrupt X Blocks.
I´d say about 100 of the former and 10-50 for the latter would be fine.
After all, its more about you getting the method done, and noone wants corruption in their world. (except you know, you´re going for a fully corrupted world)
i like the "kill 666 demons" and the "kill 1000 undead" quests. They are very much doable when you play for some time, but nothing you have to look for (like killing Tim. 2% spawn chance, i have some hundreds of hours in terraria and only encountered him once. if you´re unlucky, you´re screwed.)
Also, some spawnable monsters could get an extry achievement, like the Kingslime. That way, you´re not bound to beeing lucky.
I really hope you won´t include "cleanse the whole world of the corrution" into those achievements. i doubt that will be fun.
Same goes for stuff like "kill the dungeon guardian" or " reach wave 15 in the frostmoon event". you can´t solo that.
Maybe an Achievement called "hellevator: take 10k falling damage" (or if you manage to implent it the way that it tracks you falling from the surface to hell in one go, that would be even better)
Another one: Reach Hardmode with a Hardcore character. (but thats already kinda... forced)
I´m a bit concerened about the 100 fishing quests. I´d rather go with "fish 10 different fish" or something similar.
All in all, i´d much prefer the Achievements actually track your progress, than forcing you into doing stuff you´d never do on your own. That doesn´t mean the tasks should be easy or done within some seconds of gameplay. it just means you can track, what you´ve achieved while having played the game the way you would have played it without the achievs.
But i guess many people have a different point of view for that topic.
Liz