Here's my ideal version;
- Ascending AC, to hit
- Skills Based
- Feats for all special abilities including "class" abilities
- Some feats preclude others. You can't have "master spell caster" and "master martial combatant" at the same time.
- Spell casting for all supernatural abilities
- Roles vs. Classes. Roles are templates with guidance.
- Unified XP advancement for all characters
- Basic combat with optional advanced rules for those who like 'em. Advanced is compatible with basic.
- Spell / Ability creation system. All supernatural spells and abilities are created from this system.
- Spell system should be modifiable. Both in creating spells and for casters to tweak upon casting (range, duration, power, etc.)
- Creature - Monster - Foe generator upfront to create custom enemies. All monsters in the MM are built from this system.
- Equipment - Item - Vehicle generator (simple). All things are built from this system.
- Options for regular adjustments to AC, to hit, abilities, etc. built into the level system. Then you can flavor as you want vs. requiring magic item acquisition.
I would start with Pathfinder 2e and Fantasy Craft as inspirations. I think Fantastic Heroes & Witchery, ACKS, and Radiance d20 would have some influence. Then sprinkle in guidance on how to customize my "generic" D&D to flavor for high fantasy, gritty fantasy, science fantasy, or traditional OSR game play.
I'm not sure on the limits on "mastery" between magic and martial characters, cuz part of me would rather let feat availability sort that out on its own, by indirectly limiting how many feats you have available to pump into either area, and still have enough for other stuff to begin with. But I could see how unfettered access to both might potentially be a problem. But other than that, most of this is more or less the direction I'm going for.
I already started talking notes earlier this weekend on what sort of things I would like for such a system and already had the idea of "progression paths" planned out for once I'm done defining every feat and core component (which is still pending process). The basic idea is that everyone, including characters and creatures, would have some core components, including a race and background, character level, 1d6 (maybe 1d8, but IDK) Hit Dice per level, a Proficiency modifier and a certain number of feats per level. And feats are used to build on top of those elements. Extra hit points (beyond a 1d6 HD) would be based on the selection of a Toughness feat, which would grant +1 HP per HD per selection (+4 per HD max).
But "progression paths" would be used for people who don't have the patience for so many selections, as well as to facilitate encounter creation for GMs. All creatures would basically have a character level, just like PCs, and their Size (based on their "race") would grant additional Hit Dice and other modifiers based on their size category. So GMs just have to pick a "race" template, strap in a "progression path", and presto! Readymade encounter to go!
I'm also thinking of using 1d6 as base damage for all weapons (including natural), but modify that die type based on the weapon's properties: Light, Heavy, Martial, Two-Handed, Firearm, etc. Most properties (other than Light) will cumulatively increase weapon damage by one die type, but add some sort of caveat (requires two-hands, special training, etc.). Light weapons are reduced by one damage die type (1d6 to 1d4), but may be used off-hand without penalty and are treated as finesse weapons. Weapons for size categories larger than Medium also increase by +2 die types per category (Large +2, Huge +4, Gargantuan +6), and smaller weapons are reduced by -1 die type for Small or -2 for Tiny.
Die types world include: 1 HP, 1d2, 1d4, 1d6, 1d8, 1d10, 2d6, 2d8, 3d6, and +1d6 per type beyond.
All of this is still a work in progress, though, and subject to change depending on feedback or other stuff that occurs to me.