1. As a society, the Celts are illiterate. While they can certainly learn to read and write the languages of civilized kingdoms to the south (something akin to Etruscan and Greek/Mycenaen city-states), the Celtic religion forbids them from recording the rituals they perform - including spell casting. Years of training and rote memorization is required.
Bards and Wizards perform arcane casting through tattoos of animistic and Celtic knotwork tattoos they bear and are trained tattooists for the clan. Meditation required to activate daily spells worn as tattoos. Scrolls are Celtic knots tied in rope with gold, silver or bronze wire interlaced with hemp. Knots are untied to cast Knot Scroll.
The Wizards of the setting are called Myrddin, and simply remove "spellbook" and "scroll" from a Wizards mechanics, replace with Tattoo casting, and its all flavor from there. Myrddin live apart from clan society though serve as advisors, tattooists (work as a type of magic item granting some wondrous item or AC enchantment bonus, DR, or other attribute placed into a tattoo.)
Well, I remember seeing somewhere that written lore for Celtic stuff is Carved, and many bards would put stories into staves. For the Wizard, I'd suggest you use a slightly different system than Tattoos. Spells are carved into a wizard's staff. Scrolls would still work, but wouldn't be made of paper. Say they weigh more, are made of thin animal hide, and when consumed, the leather is turned fresh again, and can be reused.
Because the culture is illiterate, each wizard's method of recording spells is VERY different.
For the Magic Tattoos, that sounds like a 3 to 5 level PrC.
The problem I see with the Bard and Wizard is they're a bit too civilized and urban. I'd suggest that in addition to the above, give them different spell-lists. Take spells from the Druid/Cleric/Witch lists that fit, and add them into the wizard/bard class as an alternate build. Same with the sorcerer.
An alternate build is preferable to a full new class, IMO.
2. The Pathfinder Druid being more of a shifter doesn't really fit a Celtic Druid, so Druid is restricted from the setting. Druid is now a trait and applies to the Druidic caste which includes the historians, law givers, divine and arcane casters of Celtic clans. Classes that can take the Druid Trait include: Bards, Sorcerers, Witch and Oracle, the latter better fitting the idea of a traditional druid.
I'd add wizard to the list that's up there, and re-flavor a few things to fit the culture, and then alter spell-lists a bit to match the culture and society better.
3. The Fighter is restricted from Celtic society, as Celts lack the discipline and driling that fighters receive in training. The replacement class is the Clan Warrior.
Again, I'm going to suggest Alternate Builds over class rewrites. I really like the direction they took with sorcerer bloodlines, and a bunch of the other classes in PF. Make it a different build of fighter. Give them only access to light armor with medium armor (Hide) as a single exception, turn your armor training into shield training, and give them some kind of ability to compensate for the low AC, such as a duelist-like dodge bonus. Give a similar treatment to the Barbarian. List it as an alternate build.
Because Celtic warriors are known to fight as berserkers at times in combat, I was thinking of adding half progression Rage Powers, so they gain 5 Rage Powers by 20th and no Mighty Rage.
I'd pass on that one. With it adequately reflavored, there's a good chance they'll multiclass barbarian fighter if they want the rage. I'd try to give them soething new instead.
I want to introduce a new Combat Maneuver called Salmon Leap, against a flat-footed opponent, make a verticle jump to make a single attack as a standard action that bypasses Shield AC bonus, but grants an Attack of Opportunity.
Cool Idea. But why (logically) couldn't they just move their shield to compensate, and what if you suck at jumping? would it be an acrobatics roll? The height you'd need to get that reach over the shield is around a DC 12 acrobatics roll, not taking into account that they could raise their shield or angle it differently.
Instead of Fighter feats (no fighters) there are different Clan Warrior feats
I like the idea of a different feat list, though some of the fighter feats could easily still apply
Rogues and Rangers exist as special kinds of warriors for the clans, especially for stealthy and wilderness adventuring.
I believe there was a wilderness rogue build in 3e (check Crystalkeep to get the source) that may give you some good ideas on how to make rogue fit. Ranger should fit okay as is.
Charioteer is a martial Prestige Class that includes driving a chariot, and attacking from it with throwing spears.
I'm not sure a PrC is needed for this. Just make a Chariot item that can be bought and use the ride skill. A chariot isn't of much use in forests.
Other campaign ideas is to introduce a "Roman Invasion" attempt by one of the civilized militant states from the southern coastal fringes of the Celtic World.
I like this campaign Idea. You should merge the empires a bit. Combine elements of greek city-states, the roman empire, and the chinese empire maybe, and create a unique but familiar type of culture, with elements of other cultures.
OK, here's my first attempt at looking at the basic Clan Warrior Class.
Alot of that is so close to the fighter that again I say you should just make it an alternate fighter build. Maybe a fighter build with less armor training, a small level-dependent dodge bonus to ac, and give favored enemy to pretty much all the martial classes. Alot of those weapons are pretty much identical to existing D&D weapons. a Gladius is basically a short sword. I'd just re-flavor the names of a bunch of weapons, rather than create that many new ones, unless absolutely necessary. You may want to look into some sort of AC progression for everyone. With the time period you're looking at, people can't really pump up AC like they do in regular pathfinder. Frontline fighters are probably going to cap out in a breastplate, and that's if they come from civilized countries. that means a lower AC.
Bah, for my setting its ink not paint, rather than painting blue designs on warriors, its blue ink as a tattoo permanent placed under warrior's skin. And in this settings case, its the knot pattern or symbol depicted more than the ink/paint itself.
WOAD can't be used as tattoo material, just so you know. Once the stuff starts to dry, such as if you used it in a tattoo, it starts to turn white, and eventually isn't blue at all. The blue tattoos the Celts sometimes had were likely made of tiny amounts of melted copper (poisonous)
I would suggest tweaking the Fighter -
very slightly - rather than creating a new Fighter analogue class. I mean, I realise you have already, and if that's what you prefer, then more power to you. But, my perspective is, the features presented would be better suited to feat trees and the like. Maybe strip heavy armour proficiency from Fighters in this culture/land/context/setting, and give them some reasonably minor ability in its place, to compensate.
The beauty of the Fighter class
in particular - along with a couple of others, generally - is that you can use it in just about any setting, often with no tweaks, and occasionally with one or two on a small scale.
But, full disclosure now, I am not keen on adding more classes in general. To be honest, it strikes me as the last thing Pathfinder needs. If there is literally no other (or, no
better) way, then sure. Otherwise. . .
I tend to agree with most of this. Alternate builds, different 'fighter feat trees', and different spell lists are cool, and give us interesting ways to build familiar types of characters, but We don't need all the classes 3.5 had, unless they are truly unique. If what you've made looks alot like a multiclass build turned into a class, maybe it should be a variant build instead of a whole new class, or maybe you should just allow the multiclassing.