|A beautiful cake baked by Alice who is a much better baker than I am!|
Do you want to bake a cake, but you don't have cake flour? You can make your own: use a ratio of "14 tablespoons all-purpose flour whisked with 2 tablespoons cornstarch."
Do you want to use avocado in place of something or other as suggested widely across internet sites? Maybe it will work, maybe be a disaster. In baking, mostly the latter, though I once saw a cooking demo that used avocado in a chocolate mousse and was unexpectedly delicious! (It wasn't a substitution, though, it was a carefully thought-out recipe, which is a little off topic here.)
Useful website "Food 52" is currently featuring a list of substitutions from plausible to impossible: "10 Baking Ingredient Swaps That Won't Fail You (or Your Cake)" by Sarah Jampel. Most useful, it has a list of goals that you should be aware of when you substitute. Will you change the ratio of wet to dry ingredients? Will you alter the fat content in a significant way? Do you need to understand the chemistry? Really helpful!
When I remember to check Food 52, I often find intriguing suggestions. This article also talks about equivalences of various types of sugar. It mentions the frequent use of applesauce as a replacement ingredient. And lists several fermented milk products like buttermilk, for which my mother's chocolate cake substitution, milk with a few drops of lemon juice, is approved!