There have been a couple of posts of late pertaining to NPCs, their presence, and their role in the game. I was considering this quote in particular, which someone else had also reference but I was unable to locate:

"At the core of it this is my problem: this is why I want to see Gotham alive with people and culture and museums and parties and schools and celebrations and life. Because superhero stories make the most sense to me when the promises of their cities are made clear. The promise is vital..." from this article http://www.giantbomb.com/articles/superheroes-cities-and-empty-streets/1100-5241/

I believe this to be an insightful perspective about using NPCs to help reinforce the immersion factor, and started thinking about what else could be done in this context.

1st - do we WANT a realistic, or even semi-realistic urban environment where there are large numbers of civilian NPCs going about their day-to-day business? would this cause the inclusion of villain mobs to be perceived as an immersion breaker without tying their presence to changes in NPC behavior? A group of zombies is walking around - the area should become utterly barren of civilian NPCs rather quick. A group of large, aggressive men in masks with bats and guns would likely produce a similar outcome, etc.

If we do opt for a more realistic urban environment (i think we should) then here are a couple suggestions:

2nd - Align NPC total number, crowd density, gender/race dispersion, behavior, and movement patterns by neighborhood, by time, by weather (when introduced). Rush hour should see more people on the street walking or in cars than 4am. A waterfront should see more men than women, more manual labor clothes than business suits, less pedestrian traffic and more concentrated semi-stationary work/group activity, etc. The aim should be (if practical) to create a city that's alive and of which the heroes (and villains) are a part not the whole. CoH and most other games are really just urban arenas devoid of more than a few token NPCs walking the same strip of sidewalk.

3rd - incorporate behavioral effects. If a mob is present, the NPCs should avoid the mob (cross the street, walk faster, etc) or panic, or call for help. Whatever is deemed an appropriate reaction given the perceived danger of the mob. This same dynamic could/should hold true for player villains. As their infamy increases so does the intensity of NPC reaction.

4th - incorporate scatter effects. If a fight breaks out the civilians should scatter and break for cover or simply flee the scene, eventually returning to normal patterns once the conflict resolves. CoH sort of did this but was a little primitive in its approach. ONE guy walking down the sidewalk in Talos island, sees you fighting, throws up his hands, mock yells in fright, then scampers off in the opposite direction. This calls into question how to incorporate mobs into the urban context in a believable-ish manner.

5th - potentially incorporate curiosity/responsibility effects. some number of NPCs flee but only a short distance. Some might be police cordoning off the area, or police/concerned citizens helping their fellow citizens flee. Others could represent the youtube/selfie types who want to capture the event happening, so they turn and break out mobile phones to record it.

The latter could play into the 'fame dynamic' where if hero X is doing a lot of street level patrolling and assistance s/he could be captured on film by pedestrians. Done enough times in a concentrated area and the local people in that neighborhood might offer thanks or be heard discussing the hero. Again, something that CoH offered to a degree, but using NPCs at the scene to reinforce how a hero's fame spreads would reinforce the immersion by illustrating the start of the causality chain.

This could also differentiate neighborhood heroes from global heroes - think daredevil vs ironman - if someone is spending a lot of time mission-ing/patrolling a particular neighborhood then have it reflect in conversation, tips, maybe even temporary graffiti or other recognition symbols using the hero's/villain's name.

Nothing here is particularly groundbreaking, but the lack of most/all of these features contributes to a reduction of enjoyment and replay-ability. Obviously, capabilities/time/money need to be taken into account, but over time the aim should be to model and create as healthy and vibrant an urban environment as is practical.