I sure am procrastinating, and this stuff has been percolating in my head for a while. this is how it works so far. I'm semi-reluctant to post it up just because Keyblades should be MYSTERIOUSSSS and yet it helps me to kinda talk about rules where people can see SO
This is different from my first thoughts! Here is what I have. Maybe I've talked about this before? I am talking about it AGAIN
Greater Keyblades are each a shard of the χ-blade (actually also pronounced Keyblade; that symbol is used because of some stuff in Plato about the soul of the world, the anima mundi, being composed of two halves that cross themselves like the letter χ). The χ-blade was the key to the heart of the universe, Kingdom Hearts, and was shattered into 20 pieces during the Keyblade War, when that heart was opened and the original universe was sundered into the myriad worlds it exists in today. Each Greater Keyblade corresponds and holds a fundamental part of existence, and are in themselves intelligent forces of nature with their own agendas. They are roughly divided into two types: Dark Keyblades and Light Keyblades. These concepts actually don't correspond to good and evil precisely! Darkness is much like the Dark Side of the Force: it's a force of destructive power and is easily used for evil, and is indeed naturally corruptive, but it in itself isn't intrinsically evil, it's a force of nature. Dark and Light will never precisely correspond to alignments, they are forces. Bigger post on that later. Point about Keyblades is that while both Keyblades are capable of both opening AND closing things, Dark keyblades tend to be much better at opening paths, whilst Light keyblades are better at closing them. As an example: a Dark keyblade might be able to simply close and lock a door, or Hold a portal, but a Light keyblade could magically seal a door in such a way as that it could only be opened by that Key, or a Dark keyblade. A Light keyblade might be able to open simple locks or simple magical wards, but a Dark keyblade might outright Shatter the locks or break down the doors.
Lesser Keyblades were originally forged by the Order of the Keyblade before the world was sundered; they were created by crystallizing a tiny piece of the heart and soul into an empathic weapon. In this way, the Keyblade was a part of their master's will, an extension of their self. When a Keyblade Master died, their weapon remained behind, and was often either used as a grave marker, or, in dire circumstances, gifted to others to wield. In later days, as the war grew more dire, Apprentices would instead be chosen by the Keyblades of old Masters to train with, before having one of their own forged in a formal ceremony.
In the modern age, a Keyblade can be wielded by anyone; these are called Wielders, and they are still very special people. Keyblades have individual wills, and they CHOOSE who can hold them (well, sort-of; it's usually more like "Hey no stop doing that I want to do this other thing wHAT ARE YOU DOING). A true Master's blade is forged from their very soul, and this is what differentiates the Keyblade Master class from "just some dude flailing around with a magic weapon."
Lesser Keyblades are weapons in two halves; there's the base Blade, and then the Keychain. Base Blades I am still figuring out, but they are basically +1 mildly emphatic magic weapons (I'll work out rules for intelligent weapons later; every one I've seen from D&D seems kinda clunky?) usually with a theme, like Sun or Light or Water or Earth, based on the personality of the Master that created the blade. In the case of dead Masters, in some ways the Keyblade is an afterlife for them, containing a bit of their spirit. Each Keyblade also has either Knock or Hold Portal 1/day, depending on if it is a Dark Keyblade or a Light Keyblade (yes, Lessers correspond to this dichotomy as well).
So to sum up: +1 weapon, Knock OR Hold Portal 1/day, empathic
Keychains, meanwhile, are enhancing artifacts that represent the connection of a Keyblade Master (or even just a Wielder) to other people. These chains represent relationships both good and bad; some might be the result of a great friendship between two individuals, others might be born of a victory over a terrible foe, or even over a particularly spirited rivalry.
Chains are what tend to have the FUN ABILITIES. That's how you get stuff like Detect Magic, Fire damage, additional pluses, and so on. Keychains might also give the Keyblade a MUCH stronger personality, or even override the original personality; usually reflecting the personality of the connected person.
Note that for a Keyblade Master, they can temporarily convert the Keyblades of dead former Masters into Chains; using a Keyblade in this way will give other abilities. Wielders can't do this, except in the case of Greater Keyblades. With Greater Keyblades, the Greater ALWAYS overrides the Lesser and will basically turn it into a Chain.
EXAMPLE, AS THIS IS CONFUSING: If the keyblade Frolic Flame, which is just a +1 blade with Knock, were used as a chain for the Kingdom Key, a Greater Keyblade, the two together would become Bond of Flame, a +3 Keyblade that grants Fire Resistance, and allows Burning Hands 3/day, and Wall of Fire and Fireball 1/day.
NEW AND DIFFERENT THOUGHTS ABOUT KEYBLADE MASTERS, IN LIGHT OF THIS STUFF
I was ORIGINALLY going to make Masters kinda just like Elves with fewer spells and more weird shit related to the Blade, but now I'm thinking of making them a little more fiddly and keying their stuff off the Blade more. Instead of progressing Base Attack or their spell list, they can imbue abilities into their Keyblade via Keychains based on the friends they make and enemies they defeat. Not sure how I'll make that work mechanically though.