Remember the "KISS" acronym? It stands for "Keep It Simple, Stupid." The more complex we make things, the more likely they are to fail. Of course, there is always a minimum level of complexity required to give any endeavor, including games a feeling of worth. If the challenge is too trivial, then then conquering it doesn't provide any feeling of accomplishment.
That being said, a game, even robot games, should only be as complex as is absolutely necessary. Not a whit more, nor a whit less. A great example of a 'simple' robot game that turns out to be incredibly challenging, and tremendous fun, is the Die Shoot contest developed by Ishikawa-san for the Wonderful Robot Carnival events held on a biannual basis.
The rules are simplicity itself. Two robots face off with a large die between them. The goal is to move the die across your opponents goal line while keeping them from moving the die across your goal line. There's a time limit, usually 3 minutes. The robot that scores the most goals during the period is the winner.
In early versions of the game, a single point was awarded for each goal. To make it more interesting, they have instituted a variation where points are awarded based on the up face of the die when it stops moving after having been moved across the goal.
In order to win, you have to build and program a competitive robot, then develop both offensive and defensive strategies, and practice them incessantly, to the point that you can execute them in your sleep. This 'simple' robot game demands a lot of competitors. And, it return, it provides them with a real sense of accomplishment, excitement, friendship, and a ton of fun.