I think you are exactly right to start with some soy sauce. You could always tweak it a bit by adding some brown sugar, ground ginger, a bit of rice vinegar, and some siracha if you like it spicy to make a richer sauce with a definitely Asian twist. Using different oils in the stir-frying process can also help to add a bit more complexity to the veggies/meats which can save you from having to over sauce the dish later on (or it will bring out more flavor by interacting with the sauce you concocted).
As for other random fruits/veggies to grill: apples, peaches, nectarines, and pears grill well into sweeter dishes or as a condiment to a savory dish, as far as veggies, i think you can just continue on the path you are and move onto eggplants, corn and then to various fungus.
But seriously, everyone posting here as some unique tastes and a definite passion for food - I dig it! Keep up the wonderful work everyone, I'll try and add a nice recipe or two to the page a bit later.