It's not a NUC, but the ASRock DESKMINI A300 is quite small and might cover some of your needs...

The A300 is wonderful, but there aren't any Zen 2 APU's available. When the next gen APU's with Zen 2 cores come out I'm hoping to build an A300 with a Ryzen 5 APU and 32GB RAM. Seems like a dream dev machine for me.

As an alternative for the Asrock A300 but again a bit bigger there's also the beautiful but expensive Cirrus 7 incus A300, which is completely fanless[0].

[0] https://www1.cirrus7.com/en/produkte/cirrus7-incus/

Using the ASRock A300 today and already pretty happy with it (small, quiet, reasonable cost, gets the work done), though of course agree the next APUs should be a noticeable step up.

It'd've been nice if ASRock's micro-STX formfactor had taken off and gotten an AM4 variation (http://www.asrock.com/nettop/Intel/DeskMini%20GTXRX/index.as... ). It's a mini-tower bigger than NUC or the A300 but smaller than mini-ITX. Compared to mini-ITX, it takes an MXM GPU instead of having a full-sized PCI slot and uses an external brick PSU. The extra benefit on the AM4 side would've been that even a weak dGPU allows using non-APU chips, at least up to the supported TDP.

Throw in passive cooling please! Still waiting for the iBOX-R1000 to become available. Meanwhile more options are very welcome.