Here’s another vote for arbitrary symbols attached to shifted/unshifted keys. I don’t even have a keyboard yet, I ordered it a week ago 
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I suppose I could hack that effect in with a “shifted keys” layer, and not using a literal shift key in my layout. If I want to hold down shift, that’s what a special gaming layer would be for.
That is, the current state of things is actually really powerful depending on how hacky you think it is to have several layers inter-mingling with each-other. I’m thinking a lot of edge cases would be major headaches, and I’d hate to run into a couple that only bother me slightly so that I never do anything about them, if you know what I mean.
Anyways, I’ve got some pretty strong ideas of exactly how I want my keyboard to work when I get it, starting with a custom layout, which I posted about here, though I’ve made a couple more changes since that post.
I’ve got this idea of a sort of meta-layer, or group of layers associated with each-other. So, I’d be able to switch between my custom layout, with all its various layer-based “modifiers”, and the stock qwerty, with its default function layers. I don’t know if the community has already given the concept a name, but I’m calling them “modes”. It should be pretty easy to implement if I use yet another layer as my “mode switching layer”. I’m thinking of naming my modes things like “red 3” and “blue 0”. Each row of keys under the numbers would be one of three colors, the physical number keys would be indicative column headers, and only the actually assigned modes would be lit up.
What I really want, that I need help with, is the ability to change the lighting based on the current layer. If there isn’t already a generalized plugin, I’m sure I’ll be able to tear the “numpad” plugin apart to make one.
But how do I detect if (the OS’s) num lock is on or off? I’d like to base the lighting on that, as well. I decided to use a num pad for both numbers and arrow keys, the way actual traditional keyboards do. If I map a key to the literal “num lock” key, it will affect the OS’s interpretation of the “num pad” keys, right? I’m gleaning from the thread that any usually-shifted mappable key is actually implemented as a shifted key under the hood, and I’m assuming the numpad + num lock work in an analogous way.
Sorry if this post is long and rambly, let me sum up what I’m hoping to get help with. Are there already conventions for the concept of “modes”—switching between groups of related layers? Is there already a plugin for custom-lighting every layer, with provision for other arbitrary conditions, e.g. the state of num-lock?