I like to sprinkle in a bit of randomization into my games. Little surprises here and there to keep things interesting. In my previous game Zombie Tactics, characters could go off and scavenge for supplies during a battle. It offered risk versus reward - scavenging with many characters would provide the best rewards, but the player would risk getting wiped out with the remaining characters during a battle. On the other hand, not scavenging would be safer in the short run, but would forgo improved equipment and bonuses in the long run.
The simplest approach I took for Zombie Guard was a spinner wheel, reminiscent of prize wheels at a carnival. It was designed to offer temporary benefits and disadvantages. It was decorated with placeholder art. First of all, I was discouraged to spend a great deal of time making it look presentable. It was confusing too. Even worse, tapping the wheel did not feel very interactive. I like to at least give the illusion of choice.
The second iteration was just naive grid of selections. At least here, it felt like there was a choice, even though the function was just simply returning a random battle modifier. Overall this approach felt very rushed. With this implementation of the feature, it worked, but the the game was better off without it.
The latest iteration uses props from the current mission as objects to "search". There is a chance of finding something advantageous like extra ammo and landmines, at the risk of uncovering temporary setbacks like weakened defenses and stronger zombies.
It took a bit of effort to squeeze everything onto the screen. I could not write the names of the battle modifiers and the descriptions because it was just too much text on the screen. Also to prevent the player from forgetting the modifiers, sequentially I present each one being applied to the battle area with the description.
I found some good sound effects and utilized an existing particle effect. This feature now is more intriguing and cohesive with the rest of the game.
It does take a bit of time to add in all those little movement transitions of the interface, but is well worth it. Corona helps make this easy through transitions with curved interpolation options. For example, the movement of the perks is "outExpo", meaning an exponential curve that starts very quickly and ends gradually.
My general approach to game design is to offer a steady amount of progression of unlocked features, that is all polished with cool art, animations, special effects, and audio. If the game difficulty is balanced correctly, it can offer steady "fun" and keep the player engaged over time.