It's fairly straightforward, is a feat the same in kind as an ordinary feat? It's natural. For instance, you can break a rock with a hammer, or jump up onto a table. So jumping over breaking through a castle wall? Not supernatural. Superhuman, sure, but not supernatural.
Elemental didn't get any classes, it got a bunch of Themes and some feats. Ki could work beautifully that way, I think.
Perhaps un-intuitively, it opens up more opportunity for many different, flavorful, unique /characters/.
If every variation on a mounted-warrior demands it's own class, then if the game has the Cavalier, you can't play a Samurai, a Mongol, a Rider of Rohan or anything else that sits on a mount in combat without getting a new class made. In turn, that bloats the game, fuels power creep, and locks down cool stuff to define one of those excess classes into something like uniqueness, thus depriving other, similar concepts of that cool stuff.
It's prettymuch the way D&D has always done it, and prettymuch the worst way to do it.
(Yeah, I know, "tell us how you really feel...")
Which is why in HoML I made 'weak classes', each one has 3 class boons, and the player can pick a total of 3 starting boons (including ones for race, background, etc). After that boons are pretty much open. They have an 'association', which indicates basically something like "yeah, this is a druidy kind of thing" but I eschewed any RULE that forbids it being granted to any old character (and all class boons are level 1 boons). I AM going to give each class a 'feature' as well, which is intended to reinforce their role, so classes DO have a bit of uniqueness, but its a fairly small bit, so you can easily either reuse them and just not use many of their boons (so spin a build that is based on Ranger but has no real 'outdoorsman' to it) or simply go in and pick up boons of a different class (probably one in your source is best) to grab most of its goodies.
Yep, there's vanishingly little solid history on the Celts. I was into Celtic Mythology back in the early 80s (yeah, before it was cool! it was briefly cool... no really, it was ....). There's what the Romans wrote from their very biased perspective, there's legends medieval monks wrote down from there's, there's archaeological evidence, and there's the Druid revivalists of the 18th & 19th centuries, and neo-Paganism.
But, what there is does point at something between animism and polytheism. FWIW. I'd never noticed the parallel with Shinto before, that's kinda cool....
Well, the revivalists/neo-pagans basically know NOTHING, at least beyond what anyone else knows. They've invented a lot, and then later generations of neo-pagans just ASSUME that what their predecessors were telling them was somehow authentic. So now there's a huge wad of firmly-believed-in but utterly made up 'Celtic' mythology (same for Norse, etc. to a lesser degree).
The medieval monks MAY have known slightly more, but unless they were drawing from older and now lost sources, they probably didn't. After all, the vast majority of that material dates to at least the 11th Century, almost 800 years after these practices officially stopped. Perhaps there were still enough peasants passing down lore to make something of, but by that time it would have been pretty corrupt, at best.
The vast majority of what we know comes from archaeology and it is quite hard to interpret. There are many statues, ruined Romano-Celtic temples, etc. We have little idea what went on in them, mostly fragmentary and often mysterious inscriptions, and just the barest smattering of relict lore. What the Romans wrote is hardly very illuminating either. There's not much of it, its fragmentary as well, and smacks of rumor and innuendo even when we can read it.
The TRUTH is we don't even have any idea what a 'druid' was, whether it was a title, an office, or something else. Its arguable that no such thing as druids ever actually existed!
It is commonly understood in the study of religion that most belief systems start out as a sort of universal animism. It seems to mainly involve ancestor spirits, but also spirits of the land, usually has some sort of 'otherworld' aspect to it (consider Australian Aboriginal belief systems). Over time, as social power structures evolve during the shift to permanent settlements, the spirits evolve into an analogous hierarchy. In the West at least this progressed to the classical stage of a pantheistic religion, generally overlaying an earlier intermediate phase (consider the Greek Chthonic cults and the replacement of the worship of the Titans by the Olympian Gods). In Japan, for whatever reason, Shinto only shifted to a partially pantheistic mode. It kept its concepts of the spirits of the world largely intact. Truthfully, had Greek and Roman classical religion survived, it might have looked pretty much like this too in practice. All we really have are the large monumental temples, there were certainly lares and other 'local deities' in daily practice. Chinese religion evolved a bit further, but its also a weird case, being mixed with at least 3 other religious traditions. It is a bit hard to say what comes from what there.
