How about a very simple tuner - a tone generator for say, G, B, D, A, and C. I know we all have fancy Snarks and apps for tuning, but I find them to be a bit finicky. Being able to tune to a note would be pretty quick, and one less doodad to keep up with.
And as I glance to the right of my screen, I see this was suggested a few years ago. Maybe this can be a friendly nudge?
Love Strum Machine - it really is amazing. Where was this 50 years ago when I started?? Oh, right… 
Well, turn up the volume and type a chord, G, B, D, etc. StrumMachine with play the chords as you type.
I will sometimes just loop a measure with one chord in it and use that to tune my instrument if my clip-on headstock tuner has migrated elsewhere.
I’m still open to the idea of a built-in tuner for Strum Machine, though! It’s partly a matter of where to put it – I’m loath to add more buttons to the UI unless they’re really useful. But it’s something I’ll keep in mind, along with the related request of a standalone metronome.