Mushroom substitutes

I concur with Elendi's answer, especially about the dumplings in stew (but I love dumplings).

As far as stretching minced (ground) meat, I recommend you try TVP. You might be amazed how innocuous it is, and how far you can stretch meat. For an organic product that does not contain hexane (the potential carcinogen noted in the Wiki article) try TSP. It's a bit more expensive, but once hydrated, it's a whole lot less expensive than meat.

Root vegetables are the standard bulkers - potato, swede/rutabaga and the like. For certain dishes you could also add beans and pulses (but if you put beans in lasagne you may find yourself banned from entering Italy). You can also make dumplings, which are great way to make a stew-based meal more filling.

Mushrooms aren't a particularly cheap way to bulk up a meal, and their flavor and texture is such that there is no real substitute. They don't have much nutritional content anyway, so there's no point in adding them unless people really like them. In a way your question is too broad as there's so many things you can do to bulk up a meal. You can serve it over rice or noodles, you can add barley, potatoes, beans, lentils, or just add nothing and put some bread on the side.

From a flavor perspective it's not so simple. Mushrooms have umami flavor, which adds a savory note to a dish. You aren't going to replace that flavor with root vegetables or carbs. If you want to replace that flavor with something else you could use something like mushroom ketchup, tomatoes, or soy - all have umami.