I have a customer who enjoys the flavor of my cake, but is requesting a lighter, less dense cake.  If I separate my eggs, whip the egg whites, and fold them into the batter separately, will that help?  Will that alter the flavor?  I will still use the yolks.

I am guessing it would probably lighten up the batter a bit.  Sounds to me like your customer wants a cake  mix.  Lots of people do.

the problem with that is -- are you going to test a new recipe for every customer? you need to know what you know and do what you do and if people want that -- great -- if not -- holler "next!" as indy debbie used to say -- well probably still does

are you using butter? do you refrigerate your cakes?

I agree...NEXT

As the others have suggested we really need to know more - about your recipe.  How do you bake?  From scratch vs a mix?  And do you chill y our cakes well?   That will make a difference.