Cake is made from flour, fat, eggs, sugar, salt, and a leavening agent. Bisquick is made from flour, fat, salt, and a leavening agent.
Depending on the cake, either baking soda, baking powder, or both serve to leaven the cake.
Egg emulsifies the fat, allowing it to combine with the flour, which removes the flaky texture found in pies. Water allows the gluten to develop, so that it will hold the CO2 bubbles that make the batter rise.
I have made very light, and fine-textured pancakes from Bisquick, but have never tried making a cake.
If I were going to make a cake from bisquick, I would warm the mix enough to liquify the fat, so that the egg can imulsify it. I would then look at my recipe and determine if the baking soda in the mix has anything to react with (i.e. buttermilk, vinager, citrus, etc.), and if not, add baking powder to the recipe.
I think that if you understand quicbreads, and what the various componants of the recipe do, then you could successfully make a light, and fine crumb cake with Bisquick. However, that being said, it's still easier to make a "from scratch" recipe, or use a box mix.
After all, the reason Bisquick was created, was to provide the consumer with a convenient product for making quickbreads (muffins, banana bread, muffins, pancakes, biscuits, etc.). The extra attention required to turn Bisquick into a good layer cake kind of defeats the purpose of the product.
Seeeeeeya; Chief Longwind of the North