A quick look at my creative process when developing the Coronus, a sort of "solar paladin" Master Class providing the capstone to a career serving Sun Guards faction.
The Coronus can be found here~http://wyrmstone.org/x/db-show/Mechanics/Classes
The goal was to make something of a new holy warrior without stepping on the toes of Paladins or requiring a highly specific entry path. With a strong association with the sun, light-based toggles seemed obvious. I had anticipated including some fire resistance, but that was eventually folded into the new path fdor the faction, making it a poor choice to further boost in the class. Belonging to a group named the sun GUARDs, and with a mandate to defend their Goddess' intersets rather than to go out on crusades, the abilities should be primarily defnesive. The Coronus focuses on being the last man standing rather than ending the fight as quickly as possible, in fact with the gamebreaker time is almost always on your side. The faith's priciples of choice and accountability immediately suggested immunity to charms (preserving your choices) and possibly some sort of saves debuff (ensuring other must face consequences). That later overlapped too much with the Paladin and was eventually scrapped.
The requirements include some new tech, particulalry the renown requirement - one of the most role-playing driven requirements I've ever tried (jury is still out, so get playtesting, you!). The Basic Skill Mastery gives a little counter-tension to the strong combatant footing of the class as a whole. Counter-tension is important when pushing flavor rather than raw power.
The skill list includes the 4 alignment skills of the required alignment, listed directly in with the other skills for less page flipping - one of the advantages of tightly purpose-built master classes even when alignment-based.
Steps along a single path offer a diverse stack of effects for characters new to paths, while smoothly meshing with Priests and Paladins who might have already have becomes agents of the faith. This required the addition of Caster levels, which also helped the class be a good capstone for several possible entry paths.
The class uses a series of abilities that toggle based on lighting conditions, but the only ability that can be entirely
negated by poor lighting is paired at level 1 with another ability that should be always available. All the others use partial modifiers like those seen in the Lancer class - you always get some benefit, and then you get a bit more when you're in your happy place.
Providing group support, Of Your Own Will provides a tightly targeted Spell Defense, at potentially very high levels (35-40ish) to you and your team. This high effectiveness and shared protection is set squarely against it only covering a single discipline - but a damn annoying one to be the victim of
Swift as the Dawn is supported conceptually by both the high inititve and the acrobatics class skill, making it feel like a smooth progression on existing strengths.
The game breaker offers regeneration, a massively potent ability that is partially ofset by the class NOT having max vitality (trimming the pool and making it easier for foes to threaten burning through the Coronus's vitality pool).