I really want Harry Carpenter to have magical potential based off of his mother's suppressed power and his exposure to the Nightmare's magic. Imagine a Carpenter kid having a natural talent for ectomancy or necromancy, who's unknowingly opening ways past the Carpenter threshold, which is why Maggie and Mouse have been monster stomping.
Oh, and if we could have Alison Ann Summers worked in there as well, having been touched by magic as an infant in GP.
Not to mention Fitz, depending on the timing and levels of education provided. He was about seventeen in GS, but if he's missed school while living on the streets, and the school has college level courses, then he could be attending at the same time as Maggie.