Preheat the oven to 350°F and prepare a 9x13-inch baking dish.
Cook the spaghetti according to package directions. Drain in a colander and return to the pot.
Chop the cooked chicken and slice the scallions; add both to the pot with the drained spaghetti.
Place the softened cream cheese in a microwave-safe bowl and heat 1–2 minutes until soft enough to stir into the Rotel juices.
Add the Rotel tomatoes and chiles, chicken base (or crushed bouillon), and garlic powder to the cream cheese; mix until smooth.
Pour the sauce into the pot with the spaghetti, add the shredded cheddar, and stir until everything is evenly coated. Season with salt and pepper to taste 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 top is golden and the casserole is heated through; let sit a few minutes before cutting and serving warm.