To really know and do substitutions for flavor successfully, you need to study and learn about baking science and how different ingredients interact with each other. You have to pay attention to the fat, sugar, and protein (eggs and flour) of the different ingredients. If you don't and you alter the balance, your recipe will fail. Acidity plays a big role, too. Remember, a successful recipe is really all about the chemistry.
A good book to get your feet wet is Bakewise by Shirley O'Corriher. Another essential book is The Cake Bible by Rose Levy Berenbaum. Both excellent.
If you have a specific recipe and idea for a substitution, you can post the specific question here along with the recipe and people are usually more than happy to help you with it.
In general, eggs are essential. You can often sub egg yolks for egg whites for whole eggs and vice versa, but I can't remember the exact substitution off the top of my head right now. You can often sub different liquids for one another. For example, if your recipe calls for water and you want to use milk, you can often do that, but it can get tricky because liquids vary greatly in fat and sugar content as well as acidity. You can sub shortening for butter, but in my opinion the flavor will suffer in a cake. Sometimes you can sub veg or canola oil for the fat (shortening or butter) in a recipe, but other adjustments to the liquid in your recipe may need to be made as well.
Oh, it also makes a difference, I think if you are starting from a box mix. Those are a lot less touchy and harder to mess up. You can do a lot to them and they will still work because of the chemicals added to them. If you are starting with a mix and want to see some recipes, I think there is a very large thread on here with tons of recipes from a user named Macsmom. If you want a particular flavor, or just want to look at what substitutions have been made and combinations created, that might be a good place to start. If you bake from scratch, or would like to learn substitutions for scratch baking, then follow my advice above and learn about the science.
I hope that helps at least a little!