Guitar effect dry through circuit / wet dry mix?
If you have the real estate on the stomp box, I suggest a DPDT phase switch so you can switch between the mix being additive (sum) or subtractive (difference).
You might prefer volume control pots on both sides, that's pretty easy to implement with the op-amps.  Alternately, yeah, one for the mix, but I think maybe a volume pot as well.  For a low-impedence out, a small signal transformer will work wonders even if you hate what it does for the ultrasonics.