Slot Notes
A Chain: Level 25, 50, 75, 100
This is the main identity perk for the skill. It should be reliable and always relevant to someone investing in that skill. This represents - beyond just the game's expected growth of efficacy - improvements in effectiveness. More damage from weapons, less impedence from armour etc.
B Chain: Level 50, 100
This can be narrower, stranger, or more build-defining than the A chain. It only needs two versions, so it should be easy to understand. Think of it like a generic "ability" that the skill grants, maybe a fully charged Blunt attack has a chance to knock enemies prone for example.
Level 75 Choice
C1 is the generic buff. It can support many builds and does not have to be deeply tied to the skill. It gives mild character-wide benefits, but does not grant effects that would exactly define a character build.
D1 is the skill-specific buff. It should feel like the player is leaning into that skill's personality.
Level 100 Mastery Choice
C2 evolves the generic level 75 choice. Maybe it grants it some capabilities that some builds really want, maybe it just empowers it, maybe it gives it additional effects.
D2 evolves the skill-specific level 75 choice. Triples down on what the skill evokes.
Both require A4, so the player needs full mastery of the skill's main chain before taking the capstone.