I enjoy trying out different woods as much as the next guy, but for an ultimate custom concert, I would have to go with an all koa uke. I just find that the sound of an all koa uke is the most essential for me.
A dream concert uke for me would have:
-Slotted headstock w/Gilbert tuners. (I think it's a myth that slotted headstocks unbalance a concert or smaller uke. I've got no such issues with a mezzo-soprano and concert with slot heads)
-All koa body.
-Abalone rosette & purfling.
-Ebony head plate, fretboard, and body bindings.
-Bound headstock. (wish I thought of this on one of my customs)
-Bound and radiused fretboard.
-Fretboard inlay.
-Neck joined at 14th fret, long scale (16").
-Some more inlays at headstock and tail-graft.
Basically, it would be like my Glyph mezzo-soprano with a little more trimming. I did wonder if I should have ordered the Glyph as a concert, but the one I ended up with is basically a regular concert (~14.75" scale), and I have been extremely happy with it.
Now go order the uke I've detailed above and post some pictures when it's done so I can drool uncontrollably. LOL!
BTW, I'm not sure why William King wasn't mentioned throughout this thread, but maybe because he's not really building much these days. I guess the uke I detailed above probably wouldn't be built by him because I don't think intricate inlays is his forte.