While I've put my own spin on most things, much of the credit for these ideas belongs to Drashna and/or the people he derived things from. Please see his layout and userspace for lots of ideas and inspiration.
While it isn't a novel idea, I feel the need to mention it because it really appeals to me as a person that doesn't like repetitive code.
We've all come to know and love the pretty wrapper for Ergodox because it makes for such a beautiful, self-documenting keymap. In this keymap, we are building ontop of the pretty wrapper to wrap our default layers with our keys that aren't unique to the layer.
Now you can wrap your keymap with LAYOUT_ergodox_pretty_base_wrapper() and only feed it the alpha keys, having the top, side, bottom, and thumb clusters automatically filled in. If you make a key to any of the top, side, bottom, or thumb cluster keys, it will propegate to all layers that have this wrapper, or have KC_TRNS keycodes in the same location