oops, that was awkward.
It seems to me that you first need to decide what exactly you want to get in the end.
If this is a shooter, then you need to think about the behavior of enemies. Let them be faster, more aggressive. Let our hero try by sound to understand from which side the enemy is approaching him and constantly turn his head in search of an approaching target. Take your favorite game in this genre and try to implement the approximate behavior patterns of a pair of opponents, to understand what makes the game interesting with them.
And of course, in this case, it is worth experimenting with the mechanics of weapons. Maybe it will shoot quickly, but then it will take time to "cool down". Maybe on the contrary, it will gradually shoot faster and more accurately, but spend more and more charge.
All this should be related to the mechanics that are implemented by the enemies. If there are a lot of them and they run in a horde, and we decided that the weapon will shoot quickly, but cool down for a long time, then this very "long" should not become fatal for the player, but strain him enough to make him worry about timings.
If this is still more of a wander-search, tied up at recharging stations and searches, then it would be great to make the sections of the levels more colorful, so that one block of houses differs from the others in details - there are only one-story buildings, here the windows are completely broken, here there is graffiti on the walls. This will make it easier for the player to understand what area he is in and whether he has explored it. It is possible to add bonuses in the form of hidden resources - markers that immediately mark the doors of the quarter as incorrect, a power bank that temporarily increases the maximum charge of the phone, a flare, the illumination of a charging station in the area.
In fact, there can be a lot of ideas. And you should just prioritize "what the project should be" first. Then make the most basic idea simple but fun mechanics. Don't be afraid to experiment with it. It is during the game jam that you need to drive at full speed. But now you have time to try different approaches. Always ask yourself "what if" and "what do I like about other similar games" - don't be afraid to "plagiarize" anything. Everything you can think of has already been done before you in one way or another, the only question is what ideas you choose and how well you implement them. And here, if you are a programmer first of all, there is just an opportunity to shine and make a well-functioning technical part.