If the aim were to have something in my studio... I'd have plenty of options - many mentioned here.
The difficulty is rehearsals and gigging: I've got to be in some arbitrary space, get the equipment there and set up with many more time / packing / space / logistics constraints. It's a hard précis to fulfill: rugged, but small, simple, but flexible, foolproof, good quality, enough features to not need additional gear, but not so many as to make it big or overly complex.
If I had to put together a spec set (in signal flow):
* 12 channels. 8 of these could treated as stereo pairs throughout the pathway. Or even all 12! Don't have much mono. 16 channels if you're feeling generous.
* 1/4" inputs - TRS inputs for the stereo pairs would be acceptable if saving space helped.
* Input gain trim: Everything feeding this box is going to have a volume control on it (audio interfaces, synths) - Perhaps just declare "gimme line level" and be done. Perhaps just s +4/-10 switch.
* Input, post gain (if any) metering: simple, but not minimal: I need to know that I'm recording a healthy enough signal - and that I'm no wheres near gonna ever clip. So three small leds each?
* Need to be able to enable/disable inputs for recording (so there might be an input which is played to the mix, but not recorded - or an input just not used)
* Files: stereo pairs should be stereo files. mono should be mono. What if you want to gang two mono inputs as stereo? dunno... maybe this is an argument for just having stereo pair inputs!
* 44.1kHz / 48kHz and 16bit / 24bit -- only choices needed. Actually, I'll only choose 44.1k & 24 bit.
* USB stick? USB drive? - widest range of options... but I don't want to lug the damn drive around as a separate thing.
* SDcard? not as many options, but very convenient.
* I love this "on, rolling buffer, silence greater than 10 sec. gets chopped, and a new file started" thing.... but also good with press record... new file set.
* Stopping should take two buttons: shift + rec or some such - stop button or 'press rec again' is to easy to do accidentally in a small gig
* Playback is nice, because you can playback a previous recording to set house levels without having everyone standing there. Also nice to "know you got it" - but ultimately not important: I never play back on my zoom recorders.
* Generally, we are going to record everything an nice hot levels - but the mix to the house needs some tweaking. Part of me says, nope - you got a volume knob on your synth / interface - use it. And just sound check at hottest levels, record at 24 bit so you don't care, and turn down what you're sending to make the mix right.
* But that may not be practical? So a volume setting per input channel (mono or pair as above). What is this? a fader? a small fader (like those ones on buchlas)? a knob? sure... pick one. control per channel please. Okay, you want to get whacky? put WiFi on this thing and control it by MIDI over IP.... then I can do it from my gear stand.
* Mutes? maybe? can these done by doing double duty with the rec. enable mechanism so fewer buttons?
* Optional: offer an independent monitor mix...but maybe we're getting too fancy. certainly at two or more aux mixes, there are mixers aplenty on the market to handle those situations.
* Main stereo outs on two sets of jacks: balanced XLR, and 1/4". Same thing, both outputs. Let's you have an easy stage monitor. or not worry about having the right adapter.
* Metering on the main mix output. Need to see that we're sending good signal
* Optional: headphone out with same mix - and separate volume control. Just as a way of checking things before they turn you up!
* small metal box
* if controls (faders, knobs) were recessed, then all the better - or rails to protect 'em - I just want to throw this in my gear bag!
* power needs to not have the chance of falling out: if standard DC barrel connector, then please the little wings on the side that let you wrap the cord so that tugging the cord doesn't tug the barrel out.