If you think salsa isn't a candidate for the grill, it may be because of logistical concerns: How can you keep all those diced veggies from falling through the grate into the fire? The simple solution is to keep them in large slices as they grill, then chop them afterward. (I'm sure you already figured that out, but a straw man is sometimes a necessary evil.)
If you're grilling a tomato-based salsa, try skewering cherry tomatoes and grill until lightly charred and blistered. It's easier than trying to grill large tomato slices, and you keep more of the juicy stuff inside. You can grill chile peppers whole, then stem, peel, and seed (but only if you want less heat) as you would a roasted pepper. Don't worry about removing all the skin, though—the char adds wonderful smoky flavor to the salsa. Cut onions crosswise into 1/2-inch-thick slices and grill until nicely marked on both sides.
The same principle applies to salsa of all stripes, including this fantastic Grilled Pineapple-Avocado Salsa that we like to pair with grilled fish or chicken.