Prepare the pumpkin by cutting off the top and scraping the seeds and strings out. Carve a hole for the mouth and sad looking eyes (the eyes can also be drawn with a black marker). Set aside.
Melt the butter in a large skillet over medium heat. Whisk in the flour and cook for 1 minute. Add chilies and cook for another minute, stirring. Add the milk, increase heat to medium-high, and cook, whisking, until thickened, about 5 minutes. Remove from the heat and add the cheese, stirring until fully melted. Season to taste with salt and pepper.
Cut the avocados in half, remove the seeds, and scoop the flesh into a mixing bowl. Mash with a fork. Add the jalapeño pepper, onion, and 2 of the diced tomatoes. Add the lime juice and stir. Season to taste with salt and pepper.
Preheat oven to 350 degrees. Place the chips on a baking sheet and cook for 5 minutes or until warm. Pour the chips onto a serving platter. Reheat the cheese sauce over low heat, adding a little milk if necessary. Pour over the chips. Sprinkle the rest of the diced tomato and green onion over the top of the cheese. Position the pumpkin at one end of the chips. Spread the guacamole across the chips coming out of its mouth. Serve immediately.