What can I use as a substitute for hoisin sauce?
Hoisin has a few primary flavors: salt, sweetness, and umami. If it's a significant part of a recipe, leaving it out isn't really an option; you'll notice the lack of all three of those.

If you can find a fermented soybean paste that has less salt, that'd be the closest substitute, possibly with some added sugar. Otherwise, you'll have to look for other umami-rich seasonings, and again likely add sugar. See What is a good vegetarian source of umami flavour? for some ideas. You could also use things like bacon or anchovies since your case doesn't have to be vegetarian. Of course, anything with a distinctive flavor of its own isn't going to be an exact substitute (anchovies don't taste quite like fermented soybeans), but at least you'll have a sauce in a similar direction.