Oh, hey, my very first dev post! N. Jolly was kind enough to invite me onto this project from the start, so I'll comment on what I had a hand in.
Legendary Cavalier Features
While cavalier had a large issue with its fantasy being fulfilled, it also had the problem of mounted combat being... difficult in a lot of environments. To that end, I wrote Measured Gait to allow the mount to cross difficult terrain more easily and Risky Lunge so a cavalier could charge in tight spaces. Added to that was a concept I used in a home campaign a while ago, Steed Parry. Now you can use your Ride skill to have the mount take the hit for you, avoiding the worst of the damage.
Additionally I threw in some unique uses for the Commander's Aura, like Chivalry's Call that can remind your barbarian friend to stop smashing using the sheer force of your honor.
Dreadnaught
Not gonna lie, this is my favorite piece that I did. The Dreadnaught is an armored titan of the battlefield, neglecting a mount all together. The hulking weight of their armor and massive weapons control space around them and they can interpose themselves between allies and enemies, like a living shield. The downside to making the base class more mount focused is you need to replace so much more for a mountless archetype!
Marrow Rider
I knew I was going to make this as soon as I saw Commander's Aura. This rider of an undead mount exudes a Thanatopic Aura, exchanging ally bonuses for penalties to foes. Ever wanted to emulate a death knight without actually needing to be undead or evil, just creepy? Wish granted.
Steppe Rider
Given who I was working with, I knew the gun archetype would not be written by me, but there was room for a different type of ranged cavalier. The Steppe Rider gets a lot of mobility options, running circles around targets while pinning them with arrows. Eventually your pin cushion of an enemy can't move and you can even shoot their limbs off, Princess Mononoke style. I had started to move it in a more supernatural direction, but eased back on that. I'm glad I did.
Final Thoughts
All in all, this would not have been my first choice for classes to work on, but it was a great learning experience and I think it turned out great. The failings of the original cavalier have been corrected and the archetypes cover pretty much everything one can think of. My thanks again to N. Jolly for letting me assist, I'm glad I was able to contribute in a meaningful way.