forgot the shortening (1/2 cup) in banana cake. How can this be served, or rescued?

Removing fat from a cake recipe does not generally make it "heavy". Fats weigh down batter or dough, impeding its rise. In fact your basic sponge cake (sometimes called a fatless sponge) has no added fat other than egg yolks and rises quite nicely regardless. I suspect there is another problem with your cake; perhaps expired leavener or inadequate incorporation of air.

(Cake without fat can be tough, but from your question this does not seem to be be your problem.)

If the textural problems are minor enough to still serve your cake, there are plenty of options to add fat and flavor. Fatless sponges are often paired with flavorful fillings. For example, a Victoria sponge is filled with jam and whipped cream. But you can be more creative with flavors to complement the banana: pudding, buttercream, and ganache are all great cake fillings or toppings with plenty of fat and flavor.

Even if the texture is unappealing, the cake can be repurposed to some degree. Cake pops are a dessert where crumbled cake is mixed with frosting, formed into balls, stuck on lollipop sticks, and dipped in chocolate or ganache. This project re-purposes your failed cake and hides both leavening issues and missing fat.