At the risk of seeming abrupt, that what is lacking - and what would constitute the "killer app" for the Hybrid framework, per se - would be to SOLVE the problem of laying out a design - from within the framework itself - and to have it generate the appropriate CSS - to support the "generated" child themes.
Hybrid is already headed in the inevitable direction - in that increasingly (and astutely, I might add) Justin has recognized the value in expanding on the Widget mechanism to allow "insertions" at various points of "functional content". Why not be the first framework to take the next logical step and include some sort of clever, ajax-driven "template layout editor" within the Hybrid Settings area - that would allow the end-user to either "move around and resize" boxes or other designated "container units" within Hybrid that would automatically generate the correct Hybrid-based CSS for each template file - and prumulgate them to the appropriate "child theme" directory.
The first generation could be as simple as re-arranging "named boxes" within a scaled-down page display - which would somewhat mimic the current widget administration environment (or similar). An early enhancement would be to be able to stretch or expand the boxes that were being manipulated in both dimensions...
It's clear to me that the existing style sheet hierarchy would lend itself to such an approach - and, frankly, after seeing the original enhancement to the ajax-driven widget administration scheme - I thought - Well, why not provide a similar mechanism for the overall layout as well?
Certainly, if anyone can pull-off such a thing, I think that Justin could... and it might as well happen in Hybrid - FIRST.
Sure, it would mean "standardizing" at least SOME of the CSS - But that is happening already with the various widget areas... I think that for the average user - it is difficult to differentiate at times between all of the possible paths to create a similar result. In all the years I have been pouring over support forums - I am stuck by the number of interactions in which the end-user simply wants to know "what to alter where" - but really has no clue - of the relationship bewteen things.
A framework - if anything - should simply that somehow - yet ideally leave the option for more advanced users (and designers) to skirt those standardizations. We're six years into WordPress now, and it still doesn't have an end-user friendly way to build a basic 2 or 3 (or n) column design from within the environment itself - largely because of the inherent flexibility of the underlying technologies from which it is created... You can name a selector ANYTHING! - and hence, every single WordPress has different CSS to describe what are functionally, largely the same things. The ability to associate "real estate" with "formatting" and "functional content" within a single paradigm is the essential missing element.
Why not be the first to "bind" the layout and CSS to the Hybrid framework at the template-level, so that, eventually, users will be able "drag" functionality "into" the layout itself - and not just within the well-defined "widget areas"? Perhaps these Hybrid "objects" themselves could then be exported and imported as well.
it seems as it is where Hybrid seems to be going "conceptually"- (if I have sufficiently immersed myself in Justin's/Hybrid's concepts) - See "Hybrid Hook Widgets"
The "clever" part would be to bifurcate the "customizable" and "generated" resultant objects - and possibly develop some manner that they could be imported and exported such that the associated CSS went with them. Obviously, as a general approach to CSS, this would be nearly impossible - But within the limited real estate of the vast majority of WordPress sites - reconciling the applicable styling from within the "box-in-a-box" model should be possible. If nothing else - it should be possible to load it into the DOM - and build the editor as some sort of browser extension - whose resultant could be re-imported back into WordPress/Hybrid.
Anyway - that is my suggestion - For whatever upcoming Hybrid version - But before someone else does it - because whomever eventually delivers this basic capability will solve a substantial portion of the end-user inquiries - who basically want to make minor changes to layouts - but don't know (and don't want to know, particularly) the difference between a style sheet and a hole-in-the-wall.
I'm clear as a bell, from everything that I have read here, and in fact, the entire value-proposition of this site, itself - that Justin has fundamentally grasped the basic underlying issue with WordPress - and the need for a user-centered (rather than a feature-centered) set of criteria - for enhancement. The environment couldn't be "richer" in functionality - However, exposing those capabilities, and indeed, refactoring them for meaningful "consumption" requires real genius and perspective.
As far as I can tell - no other framework has addressed this basic insufficiency in user-centered design.