I like
almost everything in this thread. There are only two exceptions.
First, is that I would like the Jester painting more if it weren't only available during the event. I don't know about you, but I find that, once I hit hardmode, the only reasonable response to a Blood Moon is running as far from my NPCs as possible before they get slaughtered. So, for the sake of those like me, and also consistency, I'd like it if the painting were just unlocked for purchase when you kill a Jester, similar to how the clown vanity is sold after defeating your first Clown.
The other concern is the Frozen Nimbus...just, generally. Maybe it's just that my armour is bad, but even with a strong ranged melee weapon, the regular Nimbus enemies give me trouble, so a stronger one has me really worried. Add to that the fact it can slow you,
and that it moves faster, and I'm worried that it would be incredibly difficult to get out from under it. Also, there's the fact you're pegging the slow chance at 30%. If their attack speed is anything like their rainy counterparts, that's as good as 100%.
EDIT SECTION
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Make it rarer, but give it a summoning item.
Pile of cash to make it rain?
I'm actually cool with this. Maybe a "rain stick", that can be crafted with jungle materials early on, and is later sold by the witch doctor (who needs more stuff for his shop)? Adding to this, there could maybe be a recipe to enhance the rain stick at an imbuing station...leading to my second idea.
What if, in Hardmode, you started getting rare special rain events? Basically, a heavier "Hurricane/Ice Storm" event, where certain harder enemies spawn, and there are added weather effects, like making every surface slippery. Or there might be one similar to the "Acid Rain" suggestion, or other kinds of special storms that don't happen often, but carry other special conditions. The empowered rain stick(s) would be able to summon these storms, making it possible to farm the enemies that spawn in them.