Wish List does not guarantee a particular Fighter. You have a greater chance to obtain the Fighters from the Wishlist whenever you manage to draw A-Grade Fighters, however, it does not increase the overall chance of drawing an A-Grade Fighter.
When your Wish List is promoted you are guaranteed to get one Fighter from the Wish List the next time you are obtaining the A-Grade Fighter, but for that, you have to have all the slots filled up for Wind, Thunder, and Flame Wish List Fraction and that does not affect the chances to receive the A-Grade Fighter.