I liked the idea until the whole 'joining sides with one of the factions' part. I do really love the whole idea of things like the corruption and the hallow being at each others throats though, that does seem realistic in Terraria's sense. With the other biomes attacking mobs if they strayed into a different biome than their own like Ice vs Desert I can understand. Although I feel like all biomes should attack Crimson/Corruption as they are a plague on the world. The thought of teaming up with the corruption just seems out of place in Terraria. The Corruption and Crimson are things that want to spread over the world and no pun intended corrupt it, they want to destroy and devour everything in their path and i doubt they'd care what some human has to say to them. I just cant imagine humans working with the crimson or corruption, the literal banes of the world. I also always thought the reason the hallow creatures attack you is because they're mad you unleashed the full potential of the crimson/corruption, I don't particularly like the idea of straight up having them fight along side you, but maybe if you were to kill a lot of crimson/corruption mobs or cleanse parts of the world of it they would forgive you and could be passive unless provoked.