Thinking about the thing I'd mentioned earlier and here's what I've thought of-
Vivi and Arthur in this au are childhood friends, which helps explain why neither of them realize exactly what's going on. Since they're nearly constantly around each other, they balance each other out. Vivi's gifts of repealing spirits and such keeps malevolent or nefarious beings from going at Arthur too much. While Arthur's draws spirits of a smaller scale to their group so Vivi and Lewis can actually experience the paranormal phenomenon. When the two are apart, things go sideways pretty quick. Vivi doesn't get any interactions with anything spooky while Arthur is somewhat harassed by the spooks. When this happens it's usually chalked up to Arthur's bad luck being in play.
When Vivi and Arthur first met in childhood, their abilities were weaker than they are in their current ages. So when they'd first started hanging out, Mystery had immediately caught on to Arthur's uniqueness. At first he didn't want to get involved with the boy, wanting to keep Vivi safe from possible dangers. However once it'd become clear that Vivi adored Arthur, the kitsune decided to take Arthur under his protection. At least till his charge was old enough to naturally keep things away. The two grew up, along the way meeting Lewis later in their school years and the three become inseparable as well. Lewis in all of this is hilariously the only normal one. The tallest member is pretty mundane and is unintentionally the middle ground that fuels the Arthur attracting ghosts is just bad luck because Lewis has seen ghosts too on the trips so surely it's nothing more.
This premise would be more of a comedy type of deal with the confusion of where's the spooks Vivi and spooks need to leave me alone Arthur. It'd have the beloved ot3 of course, though the main shenanigans would be done through Vivi and Arthur.
At least that's what I'm thinking so far.
















