For Watermelon Salsa: In a medium bowl, combine watermelon, tomatoes, green onion, basil, lime juice, salt, and pepper. Toss to coat; set aside.
Place sausage on the rack of a gas or charcoal grill directly over medium heat. Cover and grill for 7 to 9 minutes or until heated through (160°F), turning once. Place flatbreads on grill rack over heat. Grill for 1 to 2 minutes or until lightly toasted, turning once.
To serve, slice grilled sausage. Divide sliced sausage among toasted flatbreads. Top with Watermelon Salsa and serve with lime wedges.