Save Army Composition and Build that Army
Now that we have the great page that shows the current army, I propose the following:
An ability first to save a few of the army compositions that you make, and second, a button to allow you to build an army matching one of your saved compositions.
The saving part is trivial. The match and build part is a bit more complex. First, it would have to dump any troops already built that do not fit with the saved composition, presumably with a warning first that its about to dump those troops. Then it would have to allocate the troops to build amongst the barracks in a way that would minimize the build time. But that's a pretty easy programming algorithm.
The result would be the ability to have a few favorite army compositions that you like to build, and the ability to create them with a just a couple of screen taps.