why can't it be a rare event that can happen at any time? (more often in hardmode though, releasing all those spirits and crap probably did something to the magnetosphere)
it could have increased NPC happiness, maybe some extra luck, and two or three unique items and possibly reduced enemy spawns
also, what do you mean by "new items"? like an item that can summon it or what?
i'd rather prefer whatever that is than hallow+snow because i despise the hallow lmao