According to The Cook's Thesaurus, Maggi seasoning sauce can be substituted with equal parts dark soy sauce and Worcestershire sauce. The popular liquid seasoning can also be replaced using Thai sweet soy sauce or even mushroom soy sauce.
Many grocery stores offer a variety of other brands of liquid seasonings, such as Bragg Liquid Aminos and Knorr Liquid Seasoning, that replace Maggi seasoning. Because Maggi sauce is a highly concentrated, thin and salty brown sauce, sticking to something with similar flavors is optimum. Most liquid seasonings are loaded with monosodium glutamate, commonly known as MSG, so use them sparingly to avoid unwelcome effects.