Ok, another valid line of inquiry is Lumina itself. What kind of place is it? It's obviously important; it's the starting location for the players. Is it big? Small? All human or a mix of cultures? What kind of rules and laws exist? How does it survive being in a darkened world? What sort of adventures can be had in the city before even venturing out into the nightlands?
I like Lumina being a city, with perhaps a river running through it. The entire haven is urbanized heavily, but with a bit of a catch. Since the place has been so insular for so long, districts and small communities have formed. This sounds a bit like some Warcraft/Stormwind stuff, but bear with me. This allows for some flavor to the city. The dwarves have their walled district, though they allow access fairly freely. They also have tunnels and underhalls that run through most of the haven, and even slightly outside, though those tunnels get dangerous and decrepit very quickly, as though the stone itself were cursed. Because it is. The dwarves thus help sustain the metal industry, crafting new metal tools and limited weapons. They also are useful for general wealth, gemstones and the like. We are taking a bit of stretch on how much metal and how many gems would be under a normal city, but that can be useful too. Maybe certain materials aren't present in the haven and must either be traded for or found by PCs. Maybe later in the campaign, the dwarves delve too deep. Not that I'd ever copy anything.
The elven district can help with food. Elves and Eladrin can both live here because let's face it, they are just 3e elves split up a bit. Anyway, Elves tend massive hanging gardens and even underground cave farms, providing wood and food for the greater part of the city. Elves are also hired by nobles and wealthy from elsewhere in the city as groundskeepers, maintaining nice green spots all over town. Eladrin, for their part, work in quiet, contemplative monasteries and universities in this district, working on magic to fuel the city's continual population increase, trying to find new ways to help their brethren continue the plant growth as more and more demand is created. Certainly a few enterprising adventurers could procure ancient texts from the nightlands and possibly be well rewarded.
Humans and halflings, as well as most half-elves, can live in the main city. A traditional if highly-compact fantasy city. Mostly stone and woodwork, though by necessity the buildings stand multiple stories high, packed tightly and somewhat chaotically over each other and bridging streets. This gives plenty of shadows and dark alleys for theives, but this district has a particularly strong police force. The town guard is mostly humans, with a few dragonborn and dwarven mercenaries thrown in. Tieflings also live in this district, not expressly outlawed, but subject to a great deal of racism. Face it, demonic ancestry tends to do that. Adventure hooks in the human district are fairly bog-standard fantasy fare. Thieves, town guard quests, organized crime, fetch-and-carry, whatever.
That leaves dragonborn. I like them being a somewhat insular community, staying mostly to themselves. They have a fairly quiet, orderly district that is really more of a neighborhood. They are few in number and carefully control their reproduction, so their community is more standard construction. Nice stonework buildings, but mostly street level with a single residence per building on the second floor. They are expert craftsmen, traders, soldiers, and guardsmen. I would say that adventures here are hard to come by, as the PC party is unlikely to be trusted without a dragonborn character. If they have a dragonborn, perhaps another group has stolen something held dear by the dragonborn, and the PC can be hired to bring it back. It's also possible that their district is so small due to being marginalized by the other races, and the PCs could be hired to find a new haven for the dragonborn, outside of Lumina.
I'd say that goblins and orcs are nonexistent in LUmina. They have to stay outside. No half-orcs either, thanks to 4e.
So Lumina is a city divided into districts. Fine, we'll cover politics of these districts later on. What protects Lumina from the horrors of the nightlands? Stout defenders on the walls? What about flying menaces? Bowmen?
I think I'll go the high-magic route and do something else. Lumina was a haven to begin with because of a secluded wizard. His tower was here (still is) and he created a number of golems to defend the place. Iron golems, bronze colossi, I don't know yet. So they built walls, running on the base instruction to defend the city and her inhabitants. I suppose the walls contain vertical bars over the river to filter out water monsters while allowing edible fish. Golems being immortal, they still walk the walls, maintaining the magical dome that keeps flying menaces at bay and defending against larger threats when required. They do their best to ignore the residents, and only give boring, mechanical answers when forced. The residents now mostly ignore them. Strangely even if they are destroyed, new constructs arrive very shortly to replace them. The wizard is apparently still active in his sealed tower, in the center of the city. Since it's been thousands of years, he's either undead or it's an automated process. The citizens typically prefer to not think about it. He doesn't affect their lives negatively, but wizards are fickle and strange, and there's no reason to provoke him.
So we've covered racial districts, some political tension, food, water, metal, wood, and protection. What's left? Well, religion is a tremendously important part of sapient life in any era. The predominant religion of an area has always defined laws, moral codes, and way of life. Of course, D&D has sort of a lax view on religion. A polytheistic pantheon of gods, with no real strife except along alignment lines, usually really only along good/evil lines. So basically a weird approximation of the God/Satan dualist relationship, split into around a dozen or so. I haven't really read up on the 4e religious changes, though I know there are a few. We'll go with major gods from Greyhawk and hack it later as required.
Obviously in a world like this, Pelor is considered the chief god of civilization. He is light and warmth incarnate; the sun that rises and falls, shining down on Lumina from afar, keeping it from the eternal night outside. Pelor is often considered the primary human deity, and that makes a lot of historical sense. Name two ancient pantheons that didn't include the sun as a major god.
Aside from Pelor, I'd imagine the other gods are worshiped in turn as well. Physical strength has always been important, so Kord is useful to fighters, warlords, and the odd cleric. Corellon Larethian is of course still patron of the elves, and probably the eladrin as well. Obad-Hai is revered in and around the elven district. I would imagine someone like Fharlanghn, the god of roads and travel, is nearly forgotten by ordinary folk in Lumina. Dwarves of course still have Moradin. As I recall, Bahamut was introduced as a deity proper in 4e, for dragonborn, so there's that.
As far as it matters for play, I would wager that in a heavily urbanized town with such limited space, small shrines would be common in homes, but for any sort of cleric services, one would need to find the large cathedral of Pelor, somewhere in the human district. This building would likely be hugely important in society, and nearly anyone in the city would be able to give directions instantly. There are smaller woodland shrines and groves for Obad-hai and Corellon Larethian, as well as large underground temple-forges for Moradin. As is typical with religion, however, I suspect that clergy of racial gods don't favor those of other races visiting their shrines. This goes for Bahamut also.
Bahamut is a special case. Most of the other gods have boring incarnations. Pelor, for instance, is typically depicted as basically Gandalf. Bahamut and Tiamat both are glorious dragons, full of astounding physical awe and wonder. The dragonborn of Lumina, despite being only a small community, have nonetheless crafted an imposing statue shrine of Bahamut in their district square. A sixty foot tall dragon, with a wingspan easily double that, carved from marble in the center of the square, stands resolute in defense of his children, with wings opened in a spectacular display. This statue has hints of dwarven and elven workmanship as well, with golden accents and dark, beautiful eyes of polished ebony.
With religion defined, there's only a few more things. I see the government of such a city being a largely appointed city council. Centralized leadership of such a disparate group of races would be effectively impossible. The council exists as a single councilman (or councilwoman) from each race, excluding tieflings. Each district also elects by popular vote, with varying levels of voter turnout, a collection of advisers to the councilmen, serving as representatives of the vox populi. Of course this system suffers from the same issues as representative republics on Earth do. Graft, corruption, oligarchism, popularity over capability, and the like all exist in varying degrees from district to district.
Councilmen would rarely bother with a non-paragon group of adventurers. During heroic work, however, the party may be approached by a district representative for work. Dependent of the flavor of adventures the party enjoys, this can be anything from political intrigue to dungeon-delving, from thieving an item from another representative to slaying a group of orcs in the outlands. Councilmen, once the party is influential enough to even matter to them, are mostly so wrapped up in their political dramas that they rarely see outside of it. Thus they mostly lend towards carefully worded dealings with the other council members, spying, and even in extreme cases, assassination. In epic tier, the council as a whole may, somewhat forcefully, request the party's help with threats that endanger the whole of Lumina.
I believe that sums up the general feel of Lumina for the time being. A generally peaceful place with of the same underbelly that any such gathering of sapients would have.
More on the nightlands some other time.