Not sure if this has been discussed before, but I couldn't find anything on it when I searched: Could Demonreach be built on the site of Yggdrasil? We know that there are roots there that go down lower than any plant puts down it's roots which seems like something that Yggdrasil could do. The leyline there has been referred to as the Well. Which brings to mind the Well of Mimir (which one of Yggdrasil's roots reaches into.) Odin taught Merlin. Merlin 'built' Demonreach. It just seems like a really good fit.

I'm kind of a fan of the more subtle option.

A Mirror Harry that is more or less still Prime Harry, only pragmatic. Willing to make the compromise for the greater good. Still hyper protective of what he sees as 'his.' Basically, a guy that is or could be Marcone's 'business partner' on the supernatural side of things. Chicago with those two running it would be amazing to witness. It also would give Harry more insight into himself than a straight up mustache twirling version of himself would.

IMO This would be a great way to show him (and us) the costs if he had followed that path without mucking around with the 'Prime' parts of the story.

PS  Harry is familiar with the 'evil overlord' list. I sincerely hope Mirror Harry or Prime Harry brings it up at some point.

Saint Patrick was also mentioned as having used the Loup Garou curse.