First of all: Congratulations on the great initiative, Paizo. I really think the idea of the hybrid classes is great and I’m sure that the final result will be awesome. I’ll try to put here some suggestions for each class:
Arcanist:
This class is lacking a bit of flavor, so I would explore the idea of a magic booded being that tries to control his powers and understand them instead of just using. Like a disciplined sorcerer. I would give him access to Oracle’s Curses as a result of him fighting his own nature and treat his bloodlines as Mysteries instead of normal sorcerer’s bloodline. He would then receive revelations of his true supernatural self.
At the end he would be much like an Oracle in mechanics but with the arcane preparation and spontaneous casting.
Blood Rager:
This class feels too much as a barbarian. My suggestion: Instead of normal mechanic for rage, make it more like the mutagen. The blood rager would drink a mixture of some herbs and his own blood that could produce different types of rage. We could have a spellrage, a battlerage, an undying rage, etc…
Brawler:
This class is lacking a niche right now. We could borrow some concepts from other sources and make some special moves for them, this would make it shine. We could make it as rogue talents: a list of special combat moves that can be selected. Example: “Fist of the dragon”: once per combat the brawler can make a devastating attack that deals extra 1d6/ 2 levels crushing damage. Other options include flying kicks, hadoukens, recoveries, etc…
Hunter:
The hunter needs to hunt a bit more. As a supernatural warrior of the wild he needs a bit more of a push. I would give him several permanent animal abilities like scent, climb and burrow speed, among others. He must be able to see in the dark, and wild shape would be welcome, even if it is of a single form. This guy must have poison use. Also, his companion must be from a list a bit different from the druid: companions that could be mounts and predators would be best. His wild shape could even be restricted to his companion form.
Investigator:
Great concept, we really needed an intellectual character that do not use magic. I used to play a vivisectionist alchemist that shared the same idea. Suggestions: Eidetic memory, Intelligence to AC, Scent with limited radius, sense motive bonus. He could study opponents as a move action to gain his intelligence bonus to hit and/or damage. We may use the Sherlock Holmes books and movies as inspirations, giving this guy a great perception and insights.
Shaman:
We could get inspiration with the warlock. A shaman has connection with spirits, but from which plane, astral, shadow, ethereal? That should be his first choice and he should gain powers from that. A shaman should be able to see into his favored plane and later, to perform some kind of astral travel into it. It would be nice if he had a spirit companion, some small fey, elemental, ghost, or other spirit. A shaman should be a bridge between the plane he chooses and the material plane, being able to hear spirit requests, calming angry spirits, and summoning them for help. Also a totem fits nicely.
Slayer:
The slayer should be the combat focused rogue, also, I favor the name “Bounty Hunter”. He seems good as it is but would give him a bit more martial prowess, otherwise, seems very good. I would give more of a “Boba Fet” feel.
Swashbuckler:
I would give him some protection from bullets. The image of the swashbuckler fighting along gunshots (none seeming to hit him) is very iconic.
Warpriest:
I would not play this class as it is. The cleric is already a warpriest, with his armor, weapons and combat oriented spells. Also, if I wanted more of a divine fighter I’d rather play a paladin. So, we need a niche between both, and then we have the Inquisitor. So, what to make of the warpriest? He should have full BBA, weak spellcasting, and more flexibility than the paladin. Options: an iconic weapon of divine energy, permanent blessings that provide passive bonuses and immunities, a marshal-like selection of inspirations, swift healing and damaging spells, permanently blessed armor and spells (action cost is a thing), ascension into a fighting outsider in the deity’ army.