I think a WG is a fine idea. I'm sure that many of us have the same desires, as well as the same ideas. Just coalescing them into something high level but cogent would be a good start and something I haven't yet seen for GPIO.
For some background... my bread and butter today is embedded development in the automotive industry. But I spent 15 years of my career writing software for UNIX and I haven't stopped. I've been running FreeBSD for various things professionally and at home for as long as I can remember (since the a.out and pre-SMP days). My primary language for FreeBSD and UNIX work has been C++ for a long time (coming up on 20 years), but my day job is nearly all C.
Hardware-wise, today's off-the-shelf SBCs are very capable devices at low cost. But there are those of us who'd like to bring the real differentiator of these platforms to bear (versus say a nanoITX x86 or amd64 platform) in our problem domains, on top of a freenix. I want a rock solid TCP/IP stack, filesystem, full POSIX APIs, real security facilities, etc. But I also want at least the minimal set of embedded functionality too: a complete GPIO API, SPI, I2C, CAN. Some higher-level hardware abstractions would be nice for everyone to have as well (quadrature rotary encoders, PWM, others). For most IoT stuff, I don't need a true real-time OS as much as I need real security (ssh, SSL, CryptoPP, etc.), robust TCP/IP and a complete development environment without a huge investment. And I much prefer a BSD license over GPL, which is one of the handful of reasons that FreeBSD remains my primary freenix target.