Preheat the oven to 350°F (175°C) and grease or spray a 9x13-inch baking dish.
Cook the spaghetti according to package directions until al dente. Drain in a colander and return to the pot.
Chop the cooked chicken and sliced scallions, then add them to the pot with the drained spaghetti.
Place the softened cream cheese in a microwave-safe bowl and microwave 1–2 minutes until spreadable, stirring once if needed.
Add the Rotel tomatoes and chiles, chicken base (or crumbled bouillon), and garlic powder to the softened cream cheese and stir until combined.
Pour the sauce into the pot with spaghetti, add the shredded cheddar, and stir until the pasta, chicken, and scallions are evenly coated.
Transfer the 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 top is golden and the casserole is heated through. Let rest briefly, then cut and serve warm.