My weirdest (and only particularly weird that i can remember) Smash dream was back in the base game days, I wanna say it was post-Isabelle? but basically there was a new trailer.
It was a new cinematic, and very fast-paced with its reveals, IIRC. The new characters just showed up, didn't get any splash screens or introductions, and there they went. It started off with the usual fare at the time, Skull Kid, Bandana Dee (I 'member being super happy at him in particular since I was a hardcore Dee supporter at the time), Isaac, the works. But the next character after them.....was ****ing Monika from Doki Doki Literature Club. After her, followed an entire wave of generic, nameless, faceless anime girls revealed one after another in extremely rapid succession, and thanks once again to the trailer having no gameplay or not giving any character an introduction of any kind, I was completely befuddled at what I had witnessed and got no time to process any of it. (sidenote, Viridi might have been somewhere in there in the middle of the trailer???? It was two years ago, gimme a break)
The final shot of the dream was me looking in horror at the recently-edited Smash wiki. Unfortunately, I can't remember exactly what was on there, so that'll stay a riddle for the ages.
All my other Smash dreams have been pretty normal all things considered, but this one in particular was simply bizarre. Kinda wish I got more weird Smash dreams, to be completely honest.