CrimsonCuttle
Skeletron Prime
Trying to get both evils in one world is incredibly difficult. No matter what you need a second world, which you either need to scour for mushrooms to make a small amount of powder to spread over a miniscule area in the main world, or you need to progress that second world all the way to the point of getting the Steampunker to sell the solution to spread.
To fix this meaningless and long grind for low relative reward, I propose that solutions and powders should be crafted into their opposite counterparts, using seeds. For example, to craft Red Solution, you use Crimson Seeds and Purple Solution, and to craft Purple Solution, you use Red Solution and Corrupt Seeds. This still makes need for a second world, but it makes it much less of a chore, and gives better use to the seeds which are otherwise pointless compared to their powder and solution counterparts.
To fix this meaningless and long grind for low relative reward, I propose that solutions and powders should be crafted into their opposite counterparts, using seeds. For example, to craft Red Solution, you use Crimson Seeds and Purple Solution, and to craft Purple Solution, you use Red Solution and Corrupt Seeds. This still makes need for a second world, but it makes it much less of a chore, and gives better use to the seeds which are otherwise pointless compared to their powder and solution counterparts.