much like my other comments on unique dungeons/resources... quite simply if heroes are not randomly generated and spawned (so there's an infinite supply), then they should spawn regularly, with the tech limiting who a faction can recruit rather than spawn on research. unlike dungeons/resources they don't need to be all spawned at game creation, because you don't need them taking out all the cities nor a whole horde of them.. even if they fight monsters or get killed by monsters randomly..
