Some people think Cowl is Harry from the future. John and Cowl have been seen in the same place at the same time. Harry would have noticed any power from John if he was a practitioner, but John is the one of the most magic savvy vanila mortles(WoJ (http://www.theonceandfuturepodcast.com/the-once-future-podcast/2014/11/16/of-podcast-ep-60-jim-butcher)). It is possible in the future John and Helen will make a deal with Outsiders for power to heal Amanda, but she dies anyway so they go back in time and become Cowl and Kumori. Or Cowl sends them back in time, as a last ditch effort, after being stopped by Harry. In other words John is just John until Cowl dies then John becomes Cowl, and the same for Helen.