Hopefully I am wrong here, but I think that the heroes unlocked via stretch goals will most likely be packaged together as a single unit. Which means that they will not be available for individual purchase. This is how they handled Zombicide 1 & 2, Kaosball, and Rivet Wars. To get additional of the unlocked exclusives you had to get another base pledge. Again this is just based on what I have seen them do in the past.