CoH actually does something similar. Spawns are created when a hero/villain passes the spawn area, according to zone level, hero level and team size.
So, if a lower level hero entered the spawning zone before you, the spawn would be according to that level. However, in KR and other non-hazard zones, there are min/max levels of spawn within the zone itself, where the general rule is that lower level spawns are closer to the entrance to the zone, the rise as you move outwards.
That, of course, means that bad guys can be clustered mainly in low-level parts of the zone, such as CoT on rooftops around the train.