While this is certainly nothing new (I've gone over these details before in my head), and I have blogged about some of them before, I think the entire picture of what I did today was overall new.
So basically, we know the fundamentals. There's an unspecified thousands, potentially tens of thousands or even hundreds of thousands, of magical girls across the world, of various types. (Including magical girls who are actually boys. Legitimately boys like Gary, transformed-into-boys, transformed-into-girls-but-identifies-as-male, and technically, those like Ruby who are biologically born male yet are transwomen who become closer to their true selves when transforming.)
However, in spite of this, there are only a few hundred that are, strictly speaking, Riders. That's because Riders are a specific type of magical girl, one technically separate from magical girls (because technically, Riders are gender-neutral even though the majority just so happen to be female), yet at the same time, intimately linked to them.
There's a bunch of requirements, but some fundamentals to meet the specifications of a Rider include being Good, Pure, Moral, having good ethics, not using loopholes of these rules (basically, no "killing everyone is Good because Death is Peace"-type loopholes that'd allow doing evil acts), not being able of killing foes, possessing some form of spirit totem (admittedly Amy comes close to breaking this rule, but she still follows it--she has contacts that act as hers), and most critically of all, being of the Light Element with their powers.
Now, beyond that, basically anything's fair game. How much they follow the above rules can vary, so long as they are still following them. (For instance, breaking someone's arm, probably not the most good/pure/moral/ethical type of thing to do. Still allowed, though, for certain Riders, because it's incapacitating a foe non-lethally.) Most of those rules are things the majority of magical girls have, anyway.
But the light element part is one of the largest universal traits. What defines 'light' can vary. Some riders tie it to Holy, some to Spirit, some to literal Light, some to generic Magic, some just to Healing, some to Life, some even to Divine(religion) you get the picture; the definition of 'light' is very loose, and as a result, not all Riders' powers are the same. Some have their powers basically have no 'light' in them all, functioning more on said generic magic; others have their powers entirely reliant and derived from the light.
Some Riders are very, very weak, barely having powers at all, while others are completely godlike. (Including, eventually, our own characters, for good reason, though they don't start that way.) Having Light, along with having a spirit totem, are basically the main two things that make a Rider be a Rider, standing out from all other magical girls in that regard.
...Yet there's more. There are hundreds of Riders in existence...but all of them have that specific tie to the element of Light. They cannot, by definition, use multiple elements...with the singular exception of the Elemental Riders, the great heroes of legend and tale. (Which our protagonists happen to be the latest incarnations of.) They are special and unique among Riders, because there are only seven (later, eight) of them, one for each Primary Element.
While there are many 'elements', only eight elements are considered Primary, fundamental, forces of the universe, each with their own unique mystical attributes. None of these attributes are universal (they exist outside the element and can be absent from someone with the element), but generally they serve as a guideline. Basically, primary elemental magic is one of the fundamental magical forces in the Rubyverse, on par with normal magic in importance. Light to show, Darkness to obscure, Fire to bring, Ice to take, Water to store, Energy to release, Earth to remain, Air to change. (More or less, anyway.)
And the Elemental Eight, as they come to know, each possess control over one element each, aside from light (or absolute complete mastery of it in Amy's case). They are one of a kind, and while not avatars of the elements, they serve as key beacons of the time. As a result, they are famously featured in many, many prophecies. This theoretically grants them celebrity status, if not for the fact that they live in a small city and generally use discretion.
Continuing along these lines, though, you can understand the reason why Ruby is the Chosen One. Beyond being a member of the Elemental Eight, she is destined to be their leader not for her personality, but for her symbolic role. (Though, arguably, which caused which? Her personality could have been pre-dictated by her eventual role.) Why? Well, while the Elemental Riders have always been destined for greatness, the Chosen One of them would have an extra edge.
In this case? Symbolism. Each primary element has an opposite. (Rather, a compliment. They may be able to nullify one another, but they also can serve to amplify each other--see also, why D.D. and Whitney fight together, and why Hannah and Sally fight together. They make good teams.) But of the opposites, none are more symbolic than the opposition between light and dark. Thus, as the only Rider (Riders inherently being of the Light element) of Darkness, she has the element considered opposite to Light as part of her.
The result? She represents not just light and dark, but the balance of all the elements, because light and dark are related to the idea of Yin and Yang in the world of magic-dwellers. Not just magical girls, but for magic-users everywhere. She's considered vital in shaping history as a result.
Note this would apply even in universes where she wouldn't exist. While the Darkness Rider may not inherently be the leader, they will always, always hold a vital role in determining the outcome of the world. For instance, in one alternate universe where Ruby never existed, Dion became the elemental rider of darkness, and Sally was the team leader. In that scenario, he was still important to things, and in a way still served as a leader, bridging into the supernatural world, but he was a supporter to the hero, Sally, while still technically being the Chosen One.
Ultimately, just because you're the Chosen One doesn't mean you're going to be doing the heaviest lifting. It just so happens that in the current iteration, Ruby does. She can, so she did. She easily could have given the role of heaviest lifter up to someone else, but keep in mind, while all this 'Chosen One' stuff sounds great in theory, it comes with a lot of baggage, baggage she wouldn't want anyone else to suffer.
Which is why Ruby will let others have their fun and contribute, even making them keep up with her skill-wise and not fall behind, but why she's always got more on her plate than the others. Not because it must be that way, not because she's selfishly hogging all the action, but because she doesn't want to see her friends get too badly hurt.
Though when it comes to destiny/fate, some things just work funnily. Earth Ridge is a strange case of being a bit of a paradox/self-fulfilling prophecy/closed loop. It's directly tied to the Elemental Riders--the Riders are there in part (large part) because Earth Ridge is, currently, the magical nexus of the world. ("Why here?" "Well, it's gotta be somewhere." "Yeah, but why of all places, here?" "...Luck of the draw?")
What that means is, basically, it's a super-source of energy which naturally attracts all kinds of attention. Yet Earth Ridge is in large part the current nexus because the riders were destined to all originate from there. (All eight of them were born and raised in some part of the small city.) And in the case of a couple, were to be drawn in after having been out. Thus, the disproportionate amount of supernatural activity in the area, including giving the reason why Lord Darkblood insisted it be his territory, and why he demanded the council meet nearby.
It also creates the self-feeding loop of power/number of heroes/villains in the area. While Earth Ridge is a city, it's a fairly small city all things considered, so it should generally not have more than two or three "big time" heroes. It has at least ten, generally more. The number of total big-time villains is actually disproportionately small compared to the number of heroes (there should be a minimum of three per big-time hero, yet there aren't thirty big-time villains running around Earth Ridge; there's a little over half that amount), but the amount of villains total is twice as large by percentage as that of a big city.
That doesn't even begin to go into power-levels; a big city would be lucky to have five godlike protectors, and since each Rider is theoretically on that level, Earth Ridge has a minimum of eight. Similarly, there should only be a small number of godlike enemies, yet half of the big-time villains are made up of them.
In short, Earth Ridge and the surrounding area is freakishly full of stuff, way more than should ever be justified, especially considering that the people outside the above demographics (that is, not heroes, not supervillains, not goons, and not supernatural creatures) are completely normal people that make up a good 90% or so of the population, meaning the concentration of freakish stuff is super-dense.
I wanted to talk about two or three other subjects, but I think this will do for now. (Well, it also helps that I forgot two of the three...)