Canon Races

Quincy

Quincy literally means 'Monk of Destruction'. They are a rather rare kind of human, since many of them where killed during the Great Quincy War, an event that now lies deep in the past. Slowly, the race is on the rise again. Quincy don't use their own spiritual pressure or particles to fight, instead, they draw them in from the air around them. Every spiritual being is leaking Reishi, and they use it.
Normally, Quincy form long ranged weapons with the help of a cross. This can either be a cruzifix, or some abstract cross like a celtic one, as long as it is made out of spiritually enhanced silver. The normal weapon of choice of a Quincy is a bow or crossbow, no Quincy would go into battle with a sword.
They also employ a magnitude of Quincy Tools during battle, like the Silver Tubes (their kind of magic) and the special arrow 'Seele Schneider', that can cut through anything using high vibration and absorbtion of spirit particles. They can't fight Hand-to-Hand nearly as good as a Shinigami, though. Also, they are limited by their human body instead of their stored amount of Reiatsu.

Hollow/Arrancar

Once a human dies, his soul becomes a Plus, a spirit that only spiritually aware beings can see. Looking like a see-through faded image of a human, they all have a chain on their chest, that slowly degenerates if they are not send to Soul Society by a Shinigami. Once the chain disappears, or gets torn out by another Hollow, the human soul hollowifes, turning into a monstrous, heartless being that feeds on the souls of the living and the dead. They are the natural antithesis to Shinigami. All Hollow wear a mask over their face and have a Hollow hole where their heart should be.
Some Hollows develop a hunger for other Hollows. Once they eat enough of their own kind, they evolve into Menos Grande, gigantic carpet-like beings with a mask. If a Hollow is strong enough to maintain his own will afterwards, he will start eating other Menos Grande and Hollow, till he evolves into an Adjuchas class Menos. They are smaller, more animal-like and way stronger than your regular Shinigami. An Adjuchas has to permanently eat other Adjuchas in order to not evolve back and loose his chance to ever evolve again. After they eat enough Adjuchas - most likely THOUSANDS of them - they may evolve into Vasto Lorde... humanoid beings with power surpassing a Captain.
A Hollow, at any stage, can break its own mask on own accord. Should they succeed - a long and painful process - their whole being will change. They will shrink down into human-like forms (though normal Hollow and Menos often look deformed), have their Hollow Hole moved where they want it naturally, and have only small broken pieces of their mask left. Also, they now carry a Zanpakuto. Such beings are called 'Arrancar'.
Menos-class Hollow and Arrancar can use 'Cero', a highly powerful energy beam.