Spirit Biome sounds cool but maybe... it should be an alt. biome, not another one to be in our world? I think that you can try to make it Corruption's/Crimson's replacer 'couse the themes are actually pretty similliar: Corruption - Sickness, darkness, death, terror; Crimson - gore, blood, death; Spirit Biome - death, spirits, terror. It can work!
[doublepost=1457963003,1457962853][/doublepost]And another thing, I don't think that adding a Predator is a good idea. Of course, in Terraria we see many 'stupid things', but that's the main reason why the mods should actually add more Terraria-style creatures and items. Ya' know, maybe more turtle types? Maybe more sea creatures like dolphins or mantas? Maybe more undead mobs? It's your mod anyways, so feel free to do it, but still... think 'bout it.