Potestas has a long history. In short, it's my ongoing attempt to define a set of game mechanics that best describes how the important things in my game world, Selentia, are resolved. The lineage of Potestas begins with Dragon Warriors, then 3.0 Dungeons & Dragons, then 3.5 Dungeons & Dragons, and now the current iteration, which is less Dungeons & Dragons and more a fantasy d20 rules system.
Well, aside from the obvious reason that I think these rules better reflect what Selentia is actually like, my current D&D campaign has been going for almost two years. In that time, the player characters have advanced from 4th to 18th character level, and we've all had a lot of exposure to the gameplay experience D&D provides. That's been an interesting and generally enjoyable experience. However, I've noticed is that some of the most important D&D game mechanics distance the gameplay experience from the game world experience I want my players to have.
For example, D&D assumes that medium to high level characters have and use many magic items. D&D assumes that characters track and manage their character wealth in discrete currency. D&D assumes that a equally multiclassed fighter/sorcerer cannot cast spells with a similar caster level as a single-classed bard. D&D assumes that spellcasters either know very few spells, or cast spells using Vancian "fire and forget" magic. D&D assumes that characters who aren't good at fighting or at spellcasting either have sneak attack or magic song. D&D assumes there is readily available magic to bring medium to high level characters back from the dead. D&D assumes teleportation is readily available for high level characters. In general, D&D trades off flexibility for standardisation in character design, and standardisation for specificity in spell design.
Well, I can't list everything ... but here's some of the major changes from the 3.5 D&D-compatible version of Potestas.
Characters increase their pool of ability points as they advance in levels, so characters can improve their more average ability scores more quickly than in standard D&D.
I wanted flexibility in character design. There are three generic character classes: the expert, the spellcaster, and the warrior. Each class has a different base attack and base defence progressions, as chosen by the character. Each class also has a bonus class feat progression and access to one feat group (combat, general, or magic). Most D&D class abilities are now feats. Many feats have changed or been removed. There are also many new feats, particularly metamagic feats.
I wanted a greater emphasis on non-combat activities in play. Each character has their own list of class skills, and can change the list at regular intervals. In general, characters have more skill points. Several D&D skills have been amalgamated. Warriors and spellcasters are more able to allocate ranks to Craft, Profession and Knowledge skills without affecting their "core competencies". Characters have a Reputation score that affects interactions with others.
I wanted to simplify resource tracking. Characters have a Wealth bonus that reflects their buying power. Experience points are spent, not accumulated. Characters do not have Hit Points, but must make Fortitude saves when damaged or take a Wound. Spells are cast using Power points instead of spell slots. There are fewer bonus types, with several of the less common bonuses rolled up into the Luck bonus.
I wanted to speed up combat. Characters make one attack roll, even if they have additional or iterative attacks. Armour Class is called Defence. You automatically add your Dex bonus to attack rolls with light weapons. Damage is a set number of hits based on weapon, Strength modifiers, and bonus damage. All characters can make a hurried full attack to have a second chance of hitting in one attack roll, and haste reduces the penalty for a hurried attack. Multiweapon fighting has been simplified.
I wanted flexibility in spellcasting. Each spell is either a "base" spell that can be heightened, diminished, and/or enhanced with metamagic feats, or is a pre-modified spell. Spellcasters know one spell for each skill point spent. The number of Power points expended casting a spell determines its variable effects, such as save DC, damage, range and area. Some spells have been made generic: for example, spells that create an energy attack are different versions of the one energy spell. Many spells grant a bonus rather than an absolute effect: for example, invisibility has been replaced with hide, which gives you a bonus to Sneak checks.
I wanted a better defined cosmology with fewer D&D elements. The World of Spirits lies over the physical world, as the ethereal plane lies over the material plane in the D&D cosmology. The souls of the dead go to the World of Spirits, and characters pass through the World of Spirits when they teleport. There are no incorporeal creatures: ghosts turn into a body of mist when they enter the physical world. There are five elements: air, earth, fire, plant, and water. There are no outsiders: they are either fey (up to large size) or giants (huge size or greater). There are no mortal or part-arcane giants: trolls (similar to D&D ogres, not D&D trolls) and other large-sized part-giants are humanoids.
In addition to the 3.5 Player's Handbook, Potestas includes material from the following Wizard of the Coast products: the 3.5 System Reference Document (SRD), d20 Modern SRD, Book of Vile Darkness, Epic Level Handbook, Oriental Adventures, Psionics Handbook, and Unearthed Arcana. It also includes material from Monte Cook's Arcana Unearthed and Dave Morris' Dragon Warriors role-playing game. I regularly read the rec.games.frp.dnd newsgroup and occasionally read the forums at ENworld, and I've been inspired by some of the suggestions and discussions in those arenas. My players have provided feedback and imput into what they like and don't like in the game, particularly my wife Bronwyn, who has more opportunity to discuss Potestas and Selentia with me than my other players.