Personal opinion answer -
lots of recipes include a filler -cereal etc. This will make the meatloaf a smoother, firmer consistency which is good for sandwiches. If you are only serving it hot with gravy then I prefer a coarser mix using only meat bound with an egg, basically an enormous hamburger.
I like to flavour it with a good heavy sprinkling of Season-All mix (which includes salt so don't add more); some coarse black pepper and a squeeze of lemon juice, along with very finely minced onion. Don't be shy with the flavours, it can stand quite a lot.
Once it's in the loaf tin, smear a thin layer of ordinary ketchup over the top before cooking.