Someone tried to gather ideas for a 4E modern game, but he received nearly no attention.
That someone would be me, and based on the thought I put into the topic I think for D&D in the modern era to work there has to be an assumed level of supernatural in the campaign, whether it's a zombie apocalypse or folk mythology being real and active behind the scenes in our world. Without something fantastic to face, it's just a bunch of sweaty guys beating up on other humans. How much fun is that?
Since 4e is really good about re-skinning things into other things, a lot of the issues just go away with a hand wave and some imagination. It's very easy to re-map races without supernatural abilities to some type of human. The difficult races are ones with teleportation (eladrin and shadar-kai) and ones with inhuman physiology (minotaurs). The former are okay if your modern setting has the sort of fluff necessary to explain away people with the innate ability to teleport or become insubstantial. The latter is okay if you're including inhuman creatures or mutants in the cast of playable characters.
Beyond that you need some explanation for power sources. Martial and psionic fit in pretty well in an assumed low magic universe and to a certain degree primal fits well with lots of Earth culture spiritualism. Druid wildshape pushes the boundaries a bit, not so much if lycanthropy is something your modern setting includes in the fluff. Arcane and divine need the right kind of fluff to make sense, even if its as simple as the Harry Potter, "Wizards have always been around," explanation.
And then equipment. I don't really like equating D&D armor with modern body armor, nor do I like the idea of modern heros running around in middle age armor so I devised a static defense bonus based on tiers that would keep modern PCs in line with base D&D AC assumptions without needing to wear armor. Then you can have body armor that carries the standard D&D armor magic properties (dwarven, blackiron, etc.) and limit a PC to using only properties appropriate for their chosen defense tier. (For example, someone on the tier that emulates Chain/Scale could only use enchanted body armor with properties valid for chain or scale.) It messes with the economy a little bit, but I think with some tinkering it could be correctable through an adjustment to treasure parcels.
If your PCs are mostly attacked with melee weapons - clubs, chains, knives, a runaway car - everything resolves itself nicely. Guns are tricky. A case was made that the range of a longbow/crossbow doesn't map to a gun accurately, but when you think about it, with the exception of a rifle, guns are only accurate up to a point. You can try to aim a handgun, but much more than a short distance away and it's as much luck as it is skill. The easiest way to model this in D&D is just pick another ranged weapon's range and map guns to that, perhaps shortbow for handguns, longbow for rifles, and hand crossbow for shotguns, assuming the bullets go farther than that, but you simply can't hit beyond long range. A more complex method would be to continue ranges past the long range of the weapon creating two more range categories and assigning a target effective concealment at the third range and effective total concealment at the fourth. It would be
possible to hit them with a good roll, but not terribly likely.
I thought about representing the "armor piercing" effect of a bullet by having guns target Reflex, but if guns play a large role in combat that makes all the work to come up with a defense system for enhancing AC generally worthless and serves to ensure
everyone will use a gun and make basic ranged attacks if their class doesn't support ranged powers. I also considered making them high crit weapons but was concerned about the effects of weapon balance caused from adding high crit to a longbow without making it need some sort of a feat to earn it. It would probably be best modeled as a feat, say, Piercing Shot (prerequisite: 11th level, Dex 15?) that transforms firearms into high crit weapons.
Edit: Another consideration for firearms - do you treat them as straight ranged weapons (and thereby using them in melee triggers attacks of opportunity), or do you treat them as virtual light/heavy thrown weapons (and thereby allow them to be used as both melee and ranged weapons)? I can see cases for both. Allowing guns in melee would mean
everyone uses guns, but on the other hand, should I really give up a free swing for shooting at someone within 5 feet of me when all I do is pull the trigger?
