In some ways I'm already on the process of creating a 5e inspired homebrew for a Feat-based d20. Not sure if I'd call this 5.5e, per se, but some of the features may hint towards a 5.5 edition (subject to change):
Race: All characters and creatures have a race. Race is a template applicable to PCs, NPCs and monsters that determines certain basic features, including Size, ability modifiers and special benefits and limitations. For creatures, race is separate from Level (see below), but may provide additional Hit Dice based on their Size (see below), and may modify their effective level for purposes of determining CR.
Levels: All characters and creatures have a level from 1 to 20 designating their overall degree of competence and power. Level 1 is a “training” level or represents children. Most of the adult population is level 2 minimum, 3 for professionals. Level tiers include: 1-2 Trainee, 3-5 Professional, 6-8 Veteran, 9-11 Elite, 12-14 Heroic, 15-17 Legendary, 18-20 Godlike.
Hit Dice: All characters and creatures have 1d6 (4) hit points, plus Constitution modifier, per level. The number of hit points gained per Hit Dice may be increased by +1 per selection of the Toughness feat, to a maximum of +4 per HD.
Size: All characters and creatures have a Size characteristic indicating their size category: Tiny, Small, Medium, Large, Huge, and Gargantuan. Size is determined by race. Larger creatures get a +4 to Strength and special physical attacks (bulrush, grappling, etc.), +2 to Constitution and +3 HD, but also a -2 to Dexterity, -1 to AC, and -4 to hide, per size category above Medium. Smaller creatures get a +2 to Dexterity, +1 to AC and +4 to hide, but also -2 to Strength and Constitution, per size category below Medium.
Proficiency Modifier: All characters and creatures have a Proficiency Modifier equal to +2, +1 per 2nd character level. This modifier is applicable to all ability checks dealing with tasks or functions they are trained on. They may also gain a cumulative +2 per mastery rank (Expert, Master, Grandmaster) by selecting the Mastery feat.
Untrained Modifier: All characters and creatures have an Untrained Modifier equal to +1 per 5th character level (5, 10, 15 and 20). This modifier indicates general competence, and applies whenever making ability checks dealing with tasks or functions they have no training on. This modifier is replaced by the Proficiency Modifier (see above) for checks in areas where they are trained.
Difficulty Class: There are eight difficulty classes: Very Easy (5), Easy (10), Medium (15), Hard (20), Very Hard (25), Formidable (30), Daunting (35), and Near-Impossible (40).
Feats: All special abilities including traditional class features and ability increases are treated like feats. Feats are divided into six list: General, Skill, Combat, Casting, Specialist, and Extraordinary.
All characters and creatures get the following feats:
- Background: Two Skill, Ability Increase and one General feat based on their background.
- Class (ability package/progression path): Two Skill, Ability Increase and two feats from any list based on their class.
- Level Feats: All characters and creatures get one feat per level from any list (since level 1).
- Skill Feats (3rd): All characters and creatures get one bonus Skill feat per 3rd level achieved.
- Ability Increase (4th): All characters and creatures get one bonus Ability Increase feat (from General list) per 4th level achieved.
- Extraordinary (5th): All characters and creatures get one bonus Extraordinary feat (stuff like Rage, Evasion, Unarmored Defense, etc.) per 5th level achieved.
Progression Paths: Progression Paths are pre-selected feat packages that allow quick progression for players who don't have the patience for individual selections. They also facilitate encounter creation by allowing GMs to simply select a race and path to instantly determine a creature's abilities.
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Other details are still in development. I also plan on stealing the Pathfinder rule that all characters simply advance in level each 1,000 XP gained (NO leveling tables!). XP from combat will probably be a factor of character vs effective creature level (modified by race package). Most of this assumes a pure feat-based development, but that component could be swapped for classes, taking some of the other elements (like new the Proficiency and Untrained Modifiers, everyone & thing has a level, etc) to build a different edition of a more D&D-esque game.