Any sort of major celestial event could be considered a threshold. The dates of the major changes of seasons; the equinoxes and solstices (Midwinter and Midsummer) as well as the cross-quarter days. A new or full moon could be one also.A Fall Line could be a threshold as well as a ley line so could any sort of large geologic feature like a fault line or the Pacific Ring of Fire. I could also see three or more ley lines crossing in close proximity forming a threshold to the area inside them.