Sub for soy sauce?

You may not have it, but Liquid Aminos would work okay. Its good stuff.

Salt, fish sauce, maybe even oyster sauce....

If you need a sub because you don't have it on hand, what do have have with flavor and punch? You could even use balsamic vinegar (smaller quantity), Worcestershire sauce (again, less), marmite., Do you have any prepared sauce with a similar base, like teriyaki? Or use the technique but go in a different direction with what you have on hand. I often use the stir fry model and then choose from one of those many bottles of sauces and condiments in the fridge to tie it together. How about a mustard sauce?