Thanks to everyone who has replied to my question, all comments are very much appreciated! Our current design sketches include:
trays of various heights, but with the same width and depth
a removable lid / closure system. The lid is optional, so that if you plan to permanently rack the trays and don't need portability, there's no waste.
stackable with the lid on, rackable with the lid off
removable inner containers in the trays, in a range of sizes which can be laid out as desired (rather than removable divider walls, which can fail). Because these are removable, they're also optional - if you don't like internal dividers at all, just leave them out. Rounded bottom corners in these to make scooping out small parts easy. Independently movable so that the entire contents can be tipped out easily when needed (this is for moving parts to larger containers when needed, and for Bricklink sellers who need to count out parts).
Once we have a first round of design renders ready, we plan to make them available for feedback. The goal is to make a storage system which suits the LEGO community, and which meets the needs of as many different builders as possible, with different sorting and building styles, and varying collection sizes.
I looked at the Kickstarter mentioned above, but I think that system isn't going to address the needs of AFOLs very well - it looks like an awesome storage system for kids, but for a detailed breakdown of parts it's nowhere near what I would want for myself.