Everyone has a fave shepherd's pie recipe. In my family, we toss a bit of summer savory into the meat mixture and it really enhances the dish.
White or yellow onion can be used in place of green onions, and any color bell pepper works. Use your favorite cheese instead of smoked Gouda, and try other ground meat. Dijon mustard works well as a substitute for spicy mustard.
I make my own flavored olive oils, which adds a ton of flavor. I also make a lot of my own stock, which doesn't contain the sodium that commercial brands do.
Potatoes can easily be cooked the night before and left to cool in fridge for ease of preparation the next day.