A weather based boss would be quite cool, especially as a totally free-range boss (well, within reason - it would be odd to see him pop up in the sewers, for example). Nightmares would be sort of harder, as people's dreams differ vastly (my nightmares are not dark, creepy, or out of focus at all, so I wouldn't connect the two at all), and pulling out an animated dream is sort of pushing it razz .
Anyway, back to weather boss - I'm not sure what it would look like. Perhaps just a general cloudiness (maybe it only comes out in day, and plunges the map areas it is over into the darkness like the night time, just reusing that). Add in thunderous rumbling and such, maybe a simple rain animation...
The boss could be the "Eye of the Storm", except this time a literal eye that appears in the darkest, stormiest part of the cloud. Anyone using AoE in this square/squares would hit him, and he'd show up as a target in any of the squares that were considered his "territory" - the center of the storm, about (which would be the "body" of the boss, so to speak). The eye itself would launch a Willpower debuff attack against anyone who it caught attacking the body. The body would use AoE "thunder" attacks (small DoT and Fear) against enemies within the body region of the storm. There would also be lightning (straight out damage - not too much, and not especially frequent) attacks against any enemy no matter where they were on the map (and if they weren't in the Body region, they couldn't even fight back against these). Wind fluffs (basically Air Fluffs, scaled to the right level) and Hail fluffs (Sleep, perhaps? or Root? Maybe a cherry fluff style kamikaze AoE attack?) would drop randomly throughout the entire map, and would actively attack anybody. Anybody killing them would be added to the enemy list of the storm itself (thus subject to lightning and thunder attacks).
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