"The game is about to start!"
The 了 here is a must. The sentence is not proper without it.
The actual phrase is 要...了，or 快要...了, which indicates that something is about to happen.
In short, 了 here does not mean "something done", but rather a change of situation/state. Please refer to the explanation here：https://resources.allsetlearning.com/chinese/grammar/Expressing_%22about_to_happen%22_with_%22le%22