There are a couple of cookbooks by Anne Byrn who has built a franchise on this subject.
Her best known book (and the one I have) is titled The Cake Mix Doctor.
written specifically on this subject.
Go to her website http://www.cakemixdoctor.com as there are several recipes and articles as well a listing of her books.
cooking.stackexchange.com/users/5431/Ray Mitchell
Using different spices, maybe a pumpkin pie spice added to vanilla cake, will give your cake a completely new and interesting flavor.
Using citrus zests will spice things up as well!
I have also used different flavored liquid extracts, added these to icing can really add that extra something special to the completed cake.
cooking.stackexchange.com/users/4888/Jginger
Substitute part of the butter with creme fraiche - say, 2oz creme fraiche for 1oz of the butter.
cooking.stackexchange.com/users/5800/Debbie A
All answers provide great ideas but I'll take Jginger's spice recommendation a step further.
I often use what I call "subtle exotic" spice combinations which is simply a fancy way of saying unusual or ethnic-influenced combinations.
For example, Black pepper with dark chocolate; chili pepper and cinnamon; or cardamom.
These types of spice combinations aren't typical in a cake and will provide a deliciously memorable flavor for your guests.
A recent unusual combination I just tasted was a spiced gin cupcake.
The baker used a basic cake recipe then infused spices and gin into the batter and frosting.
A boxed cake could be that basic recipe.
cooking.stackexchange.com/users/5027/Spice Sherpa
When I make box cake mixes, I use vanilla almond milk instead of water and olive oil instead of vegetable oil.
The vanilla
almond milk gives it a hint of sweetness and the olive oil makes it fluffier.
And then a add a touch of cinnamon and nutmeg.
cooking.stackexchange.com/users/6499/AtlasRN
Is there anything I can add to/change about a store bought white or yellow cake mix to take it from ordinary to extraordinary?
How can I bolster up a boxed cake mix?
cooking.stackexchange.com
<baking><substitutions><cake>
cooking.stackexchange.com/questions/14235
cooking.stackexchange.com/users/5696/EmJ
Anne Byrne has a couple of cookbooks on this subject, but as a general rule you can use different spices and liquid extracts, citrus zests, chocolate and alcohol to spice up a cake. Add extra spices and creme fraiche instead of the butter.
Try the cookbooks by Anny Byrn. Add extra flavours like pumpkin pie spice, citrus zests, cinnamon and nutmeg. Use creme fraiche instead of some of the butter and almond milk instead of water.