The ultimate play out of Shaggy is that Harry soulgazes Shaggy and Harry becomes Shaggy’s boogeyman.
Not so far fetched - Shaggy eats wizards magic, what if does it via a Soulgaze, forcing it on Wizards as a conduit to their magic and their life force. Instead Harry takes a huge bite out of a stunned Shaggy.
Harry has already seen the real Shaggy through his third eye and whilst it is still affecting him he survived it. We know from Battle Ground Soul Gazing Harry makes Monsters crap themselves.