Hi Naril, thank you for your interest. 

The driving force for the characters getting together is the fact that they become imbued during an incident involving supernaturals and a group led by a Wayward Hunter. As the newly imbued, the characters will have to make sense of the supernatural threat and the savagery of the Human monsters. 

Yes, there is an overarching story, currently I'm planning 2-3 self contained story arcs hopefully with satisfying resolutions.

As for the character sheets, I'm leaning towards the semi-free form, as to not alienate the people unfamiliar with the WW system or who don't own the books. The edges and powers will be pretty canon.