The goldfish analogy seems about right. I unlocked almost all heroes first time around. Playing my second time around in Breakdown i didn´t come across any of the already unlocked heroes but now i´ve passed through level 7 heroes are spawning quite frequently in friendly enclaves. Currently at level 9 and there´s a bunch of eager heroes scratching the fences to get in. Since level 7 i´ve seen the Believer, Dead Man, Mentor, Rebel, Mother and the Killer. Kind of inclined to think that´s it´s a higher level thing?? Brilliant game btw.