My players and I ran through the third scenario in our world-choosing spree a while ago, and while I will talk about it later (it is a distant area in the World of Prodigy), we ended up choosing the first setting, the demi-monde, set in the city of Tashkent with fantasy-horror themes.
The engine that I used for all three is Lamentations of the Flame Princess, one of the more popular retroclones around. However, its fixation upon traditional adventuring activities within a quasi-medieval setting and the utter rigidity of its class system left something to be desired when running a game set in modern times.
I'll talk about the economic system I'm using in the next post, so here I'll talk about the game rules I changed to adapt LotFP to the setting.
The largest changes are to the characters. In a modern game, character skills are both much more disparate and more evenly distributed between the characters - you can have characters knowledgeable with forensic science, arson, gun technology, and driving. Trying to list each potential skill leads to the kind of dysfunctional complication one finds in GURPS, so I've left it up to my players to define what skills their character possesses, giving them only a limit to the number of skills they can learn. Furthermore, each character would be expected to have a variety of skills, not just the specialist/thief-type class, and the knowledge available would often be independent of their particular specialty (the fighter character could easily know about electrical engineering while the specialist might be an expert gunsmith), which makes my previous approach, Alexis' sage ability system, not immediately workable.
My current approach, which still needs more work, splits my four classes into pairs.
Fighters and Specialists are mundane individuals who were drawn into the demi-monde incidentally and are two halves of the same coin.
At first level, each class chooses 4 skills. Fighters must choose 3 fighting-related skills (more on that in a bit) and specialists can choose no more than 2 fighting-related skills.
At each subsequent level, fighters can improve two fighting-related skills or a noncombat skill, and specialists can improve two noncombat skills or one fighting-related skill.
Currently, the fighting-related skills are different groupings of weapons that are used in a specific way. They include: Wrestling, Bludgeons, Knives, Thrown Objects, Pistols and Stunguns, Rifles, Automatic Weapons, and Specialty. With the exception of the Specialty skill (which provides training in a single, unique weapon), each skill provides an increased attack bonus whenever using the appropriate weapons - a character with two ranks in Knives would add +3 to their attack rolls when using one (since every character begins with a +1 attack bonus).
A noncombat skill is any skill that does not immediately impact combat. Depending upon the skill, each point invested increases the odds of success by 10-17% (mainly this determination is whether the skill in question is dependent upon the character's abilities or independent of them. Dependent skills require rolling under the appropriate ability score, increasing that ability score by each point in the skill, while independent skills requiring rolling 1d6 and hoping to roll equal to or less than the number of ranks acquired).
Witches and Luminaries are people blessed with profane or divine powers and have sought the demi-monde out for their own purposes. While they receive no skill points, they are deeply knowledgeable about the occult, religious lore, and so on.
The last major hurdle to handle is that of firearms. I've attempted to maintain the weapon damage scaling of LotFP to firearms, which makes them particularly lethal.
Pistols and autopistols deal 2d4 damage.
Rifles and automatic rifles deal 2d6 damage.
Shotguns deal 2d8 damage.
Sniper rifles deal 2d10 damage.
Automatic weapons attack spaces, not creatures, and all creatures within the targeted space must save v. Breath Weapon or take full damage (they take half damage on a successful save).
I've also created a Sanity mechanic similar to hitpoints, but sanity is only recovered through engaging in life-affirming activities - playing soccer, admiring art, getting drunk/high, watching a movie, etc. I'll talk about that more, later.