A retro-clone to the old Masque of the Red Death and Other Tales book.
For those who don't know, MotRD was a supplement to the Ravenloft setting. Instead of taking place in the world of Ravenloft, it took place on Earth, in the Victorian Age of the 1890s. They replaced all the classes with modern equivalences (Soldier = Fighter, Adept = Mage, Mystic = Cleric, Tradesman = Thief). Non-Weapon Proficiencies that was an optional system and precursor to the Skill and Feat system of the 3e era, was an integral part of the overall system. Percentage-based Thieves' Ability were removed and folded into NWPs. Vancian spellcasting gave way to Spellcraft and Spiritcraft tests. The system downplays the need for Medieval weapons and armor by making guns powerful and limiting the knowledge of archaic weapons no a more specialized art. They made all sorts of changes to the core rules to better reflect the era. The problem with this rulebook is that it is not a standalone rule set, but a rule supplement.
On top of changing the name and a number of key details (like renaming "Non-Weapon Proficiencies" to "Skills"), I would came the game into a complete, standalone rulebook. I would merge Adepts and Mystics into a singe class, with some more class options. I would likely remove the to-hit system, in favor of a 3e-styled AC system, as armor is that that prominent. I might make Hit Points static, with a few bonus HPs per level.
I would change the setting to have The King in Yellow (less H P.Lovecraft, and more Robert W. Chambers) in place of iconic The Red Death figure. Countess Elizabeth Báthory de Ecsed (aka the Blood Countess) would be a prominent vampiric figure in the book, akin to how Count Dracula (Vlad III the Impaler of Wallachia) is always the go-to guy for everything vampiric. Not being bound by Hollywood's views of what Vampires are, they will not die if you stab them with a wooden tent stake (that only pins them to the earth) and are free to walk out in the sun (the disintegration Vampire was started in Nosferatu - an early knockoff Dracula).
In a nutshell, it would be a revised and complete version of Masque of the Red Death, but with stuff that would make it feel like its not a complete ripoff of it.
Another one I have been considering, is a setting inspired by Dark Sun, with some Barsoom-ness thrown in.
The rules would largely be the same, but I would change things to make them work better or speed-up gameplay. As I what to downplay the need for hot, bulky armor, I would add a progressive defense system built on Dexterity and level bonuses - basically, one gets better at dodging blows. I would rather have magic work like the Psionic system, where the characters use points and to-hit rolls, and Psionic powers (if I use them at all) would be innate abilities then as acquired spell-like abilities.
Instead if it being a generic D&D world that was ruined by overuse and abuse of magic, it was once a high-tech world that fell into decadence and the overuse and abuse of natural resources. The technology of the past could control the enviroment and even convert matter into energy like the replicators in Star Trek: TNG, but they misused it for their own comforts. All "magic items" found are actually high-tech devices, but they are so strange and alien in appearance, they are seen as supernatural in nature. There are powerful god-like entities in the form of strange glowing orbs (think Loc-Nar from Heavy Metal the movie). They are machine brains form the past who were built to serve man, but rule over people like insane god-kings. They are formed around temples and use their priests to control people. By themselves, they are defenseless, but they grant powers to those who serve them well. Those who are granted their powers are called Sorcerers.
You will not see any Tolkienesque races or mythological creatures in this world. Creatures would have a Barsoomian look about them, with multiple limbs and almost alien features. People would travel under the sun bundled-up like Jawas or Tusken Raiders to survive the heat. But homes and communities have ways to escape the intense heat. They are still hot inside, but they are remarkably cooler then the extrema heat of the outdoors. Once inside people remove their grimy, dusty, sun-baked robes. Underneath their robes, they have leather straps and loops to hold their gear. Beyond that, they are nudists, and the art will reflect this. Like in Dark Sun, everyone would be at least a first level fighter. Dueling and horror would be a normal facet of life, and combat is geared around highly agile maneuvers.
In a nutshell, this would be an mature Dark Sun clone, but without all the shoehorned high fantasy elements, and few, if any psionic powers.
Please don't mistake this for an actual project. I have more creativity then the energy or writing skills necessary to draft an entire rulebook based on my ideas. This is more of a wish-list of what I would love to accomplish if I had the ability to do so.