Oh no! Harry is not a descendant of Merlin. There goes my spiffy theory.
In all humor Butcher describes his job as telling very long lies and then asks if you would trust him. Here's the thing about calling your self a liar. Why would I believe anything he says? I'd put my WAG on the back burner and wait for events. If Merlin is Harry by the way, he would be the start of the bloodline not a part of it. He can't descend from himself.
So at least a year before the next Dresden Files novel, which will make it 3 years and counting since September 2020's Battle Ground. How long before the next novel that's planned to push the overarching narrative along?
He actually tweeted The End in 2019 before deciding to split the book.
You missed that Etienne The Enchanter was a pal of Merlins, he created Bob’s original Sanctum. Thus Bob was put to work initially for ‘good’ and this shows the Original Merlin was still alive in the 1200’s, much later than thought, or at least was time travelling.
You also missed Harry’s visitation by Malcolm was not Malcolm’s ghost, suggesting Malcolm DIDN’T die by violence. Was it instead some ascension or apotheosis on the part of Malcom to a higher state of being?
Here goes Butcher rewriting history, again.