The main issue right now is that unlike the normal world gen there are no repeated biomes to the desert and forest which means when the aether or corruption spawn it will always spawn in a biome that is unfavorable, like the snow or jungle, which makes it tedious for us as it requires the player to make their own artificial biome. To counter act this I suggest the following:

• Add a secondary desert and forest so the hardmode biomes can spawn in them.
  
  
• Changing the way the hardmode biomes spawn: instead of a giant V make it a vertical line (spawning evil biomes on the secondary desert and forest) which makes it easies for world gen and for us as we can contain them easily. This will make it so that the evil biome doesn't accidentaly cross an unwanted biome and helps with compatibility like the calamity mod which brings the astral infection.
  
  
• Similar to the bee dimension, changing the hallow to a dimension would be great, making it accessible via a key dropped from the wall of flesh and a portal which can spawn in the overworld or be crafted (this could require the player making progress in hardmorde therefore introducing new gameplay mechanics, which makes the mod's playthrough stand out). The corruption could extend all the way to the underground or hell in a vertical line. The hallow could start spreading through the portal which keeps the original feature intact and also helps the player make their own aritificial hallow biome, the corruption/crimson would spread on its own after it gets extended to the underground.