Preheat the oven to 350°F (175°C) and grease or lightly spray a 9x13-inch baking dish.
Cook the spaghetti according to package directions until al dente, then drain in a colander and return to the pot.
Chop the cooked chicken and slice the scallions, then add both to the pot with the drained spaghetti.
Place the softened cream cheese in a microwave-safe bowl and microwave 1–2 minutes until soft enough to stir; alternatively warm gently on the stove.
Add the Rotel (tomatoes and chiles), chicken base, and garlic powder to the softened cream cheese and stir until smooth and combined.
Pour the cheese-Rotel sauce into the pot with the spaghetti, add the shredded cheddar, and stir until everything is evenly coated. Taste and add salt and pepper if desired.
Transfer the pasta mixture to the prepared baking dish, spread evenly, and sprinkle the crushed Ritz crackers over the top if using.
Bake for about 30 minutes, until the casserole is heated through and the top is golden; let sit a few minutes before cutting and serving warm.
