I was just thinking that since the Corruption and the Hallow are the "stars" of Hardmode, that the Corruption shouldn't be added to the world until after you enter Hardmode. Instead, replace the Corruption with another, similar biome. Like a Swamp or something. Something that's thematically similar, but doesn't spread like the Corruption and Hallow. EDIT: Ended up making some pics to show what I was talking about later in the thread. Added them here also.