Baked Chicken Spaghetti Casserole Recipe

I have to give credit to the folks at CDKitchen for this recipe (see here). I made this for dinner Tuesday night and it was very good. I'm looking for ways to "improve" it, so if you have ideas, please post them here.

The recipe says it takes two hours, but I don't think it took me that long and I'm slow. I'll post the full recipe here with my alterations.

Ingredients:
3 tablespoons margarine
1 cup celery, chopped (we didn't have it, I thought about water chestnuts, but didn't have that either)
1 cup onion ,chopped (I used 1 large onion, didn't measure)
3/4 cup green pepper,chopped (I don't like green peppers)
1 teaspoon flour (I used about 3 tsp)
1 1/2 cup chicken broth
1 cup milk
1 can cream of mushroom soup
black pepper
2 cups chicken breast cooked and cubed (I boiled two chicken breasts, didn't measure)
12 ounces spaghetti cooked and drained (I didn't measure)
1 cup Mexican blend cheese ,shredded (again, didn't measure)
buttered cracker crumbs (we had garlic & herb)

(I started the chicken cooking and the spaghetti. I guess the folks at CDKitchen just assume. Once the chicken was ready, I started the sauce cooking)

Sauce: Cook celery, onion and green pepper in margarine. Stir in flour, chicken broth and milk , cook until thickened and bubbly. Add mushroom soup. (My goal was a thick sauce)

In a 9x13 baking dish, layer a layer each of the sauce and spaghetti. Repeat layers again. Top with cheese and cracker crumbs. (note it didn't say to add the chicken. I made a layer of spaghetti, then a layer of chicken, salt & pepper to taste, then repeated. Then I poured the sauce over it all).

Bake at 325F for 45 min or until bubbly. ( I cooked for 30 minutes, then added a layer of cheese and a layer of cracker crumbs and cooked 15 more minutes).

I thought about adding some chopped broccoli. I would have like for the finished product to have more color, maybe some green and red. The broccoli would add flavor and green, not sure what for red. I also like crunchy, celery would have added some, so would water chestnuts. I'd like to find something else.

I shared the recipe with a friend who is vegetarian. For her, I suggested portabello mushroom instead of chicken. I'm sure there are many variations.

Any other suggestions?

P.S. This made 6 servings. I eat the last leftovers for lunch today...