My favorite is a butternut squash Mac and cheese, I set aside half the cheese sauce then add broth, white beans, sautéed peppers and onions, corn, and make tortilla soup.
Different enough but very easy.

When i am cooking a bunch of chicken breasts or boneless skinless thighs, i make some extra. A chicken thigh reheats quickly, on a roll maybe, or in a tortilla with some leftover salad. I can eat it in the car if i need to.
Or, first night is grilled chicken, rice and veg
Second night is a big salad with grilled chicken and garlic bread, or chicken tacos or chicken pasta. I frequently skip a night in between to make it seem less leftover-y.
Hummus makes a nice “glue” to keep everything from falling out.
My kids will eat any pasta leftovers
Any stir fry

Meatloaf
Lasagna
Burritos
Eggroll in a bowl
Stuffed peppers
Chicken fried rice
Breakfast bake
Thai curry chicken
Baked potato soup
Beef on weck but just the filling

What dishes make the best leftovers just heating up? I'm running out of ideas and everyone in my family wants leftovers for breakfast now. So it has to be a quick re-heat.

So far I have made:

Soup
Grain veggie meat bowls (very stew like)
Lasagna
American Goulash
Jambalaya
Red beans and rice
butter chicken over rice
meatloaf and meatballs

I am desperate for new ideas.

Depends on the leftovers.
Mashed potatoes could be used for cottage pie
Spinach/artichoke dip makes a nice pasta casserole
Rice or other grains-bowl or stir fry
Roasted veggies-frittata, bowl, pasta, or salad
Meats can be added to salad, stir fry, salad, pasta, or soup
Cheeses can be added to pasta, frittata, grilled cheese, mac and cheese, or frozen and used to make fromage fort

Omelets and frittatas.
I also plan for using my leftovers. Salmon to salmon salad to salmon cakes, and stir fry to okonomiyaki for example.

Anything you can heat up and eat on naan bread: gyro meat or shawarma or souvlaki.

Put it us a wrap with cheese and salsa...any kind of meat , veg or grain.

Roast a chicken.

Chicken salad
Chicken enchiladas
Chicken pot pie
Chicken and dumplings

Left over short ribs

Tacos
Pasta sauce
Stir fry with bok choy and mung bean noodles
Dumplings
Rice paper rolls with shredded cabbage red pepper sliced cucumber sliced avocados

Pork shoulder in the slow cooker. Sandwiches one night, tacos or quesadillas another, or bowls.

Stir fry
Quesadillas

I only have a few nights a week to cook, but usually don’t enjoy heating up leftovers. What are your favorites quick dishes that incorporate leftovers? Some of the obvious /easy that have worked for me:

Fried rice
Chicken or salmon salad
Soups (veggie or chicken noodle)