I heard that there's going to be something from Tangled, what if it's the "Beautiful Bullet H-E-double hockey sticks
@Cenx mentioned? The Hallow doesn't have any type of boss yet, except for the biome mimic, and Corruption/Crimson have those too, so a boss for the Hallow makes sense.
Journey mode. I think it will be epic. I do spot a flaw though. How will you get anything crimson or corruption if you have the other one? One of the Devs said that there will be a way to have Crimson as well as Corruption, and vice versa, but the key of night works depending on the evil biome. Will you just not be able to, or will you have to find a naturally spawned one?