Funnily I was thinking about a very similar layout a week ago. I though started with a 60% layout and wanted only four rows like 40%, but with all columns of a 60% and dedicated cursor keys. The latter caused the design to gain that additional column typical for 65% keyboards.
So here are my designs on this topic:
Most similar to your design, just without dedicated Raise and Lower keys around the spacebar but instead a standard 6.25 spacebar:
The same again, just with dedicated Lower and Raise keys and split space:
Having seen Zambumon’s Tokyo66 on the Nautilus Nightmares page, I couldn’t stop thinking how an arrow up key (and the according keys below) between /? and the right Shift key:
But actually that arrow keys position works nicer with a HHKB style setup as the right arrow key is on the edge of the key space, so I also made a HHKB style design without that additional key column:
And as a variant, here’s also a 1u right Shift plus an additional 1u key, e.g. a function key like the HHKB, because 2u Shift key caps are hard to find:
I do not intend to do a GB or similar either, but I intend to publish anything I needed for building one for my own needs as Open Source. Actually I already have a local git repository with all I did so far (clicking around on http://www.keyboard-layout-editor.com/ :-), but I haven’t found a nice name yet (not even sure if that’s 50% or 55% :-), so I haven’t created a project on GitHub for it yet.