Utility.
Everybody needs it, even if it isn't as sexy as a Freq'y Fuzz. Alas, anything deemed not sexy is not going to sell well. Yet, not everybody's into fuzz, and I've even come across people who proclaim to find overdrives to be boring.
A simple EQ, BMT... the "UnderDrive" .
Boost. Who doesn't need a boost once in a while?
An EQ Boost...
...
I fell asleep before being able to post the above, Cat woke me up for her breakfast and now there are all sorts of great suggestions!
While Fig has a point to limit options, I really liked Benny_Profane's Dist+/250 suggestion, 'cause the same board could also be used to build a MicroAmp. As JamieJ said.
VOL GAIN and a SWTC.
Lamma is another good'n, and again could add the SWTC, or an input cap blend as a "Tone" control. I think keeping tone simpler than a Muff's is a good idea — not that the Muff tone-stack is terribly complex, but just thinking parts count.
As much as I love fuzz, I don't think it's got enough broad appeal. I've got a friend who's been playing guitar for half a century and just this past year he finally asked me what fuzz he should get — he's NOT an anomaly; lotsa people just aren't into fuzz.
Mind, some people find ODs to be too bland.
RAT — been building for a few years (still a noob), but only recently built my first Rat. Again, not enough broad appeal, IMO.
Good points have been raised about the Vemuram Jan Ray and other expensive bootiki types that would draw in a noob.
Also suggested was a one-off exclusive to PedalPCB type circuit mashup — this would be great, but it still needs to have some connection to a famous hard-to-find expensive drawing card, ehr PCB... BJFE? Most of Björn's stuff is on a medium to high complexity level. ZVex Channel 2 w/ input blend-cap?
Rambly post, nuttin' new — 7am, Need coffee...