So I am working on this top down shooter game which at first was supposed to be an RPG, but I now decided to make it a simpler shooter. And the problem is that it's not fun. All you do is run around shooting zombies, leveling up and collecting new weapons. I need to make it more challenging, but I have no ideas. I also need to make so that the player would feel rewarded if he plays well, but I don't know how to do it well. 

If you guys have any ideas for game objectives, or fun game mechanics that would fit my game, please share it with me.

You can see the footage of my game here:

Doesn't really look like top down. 

I am not a fan of the zombie genre, but why not make something complete different, or did you browse the projects people have posted on this very site to get some inspirations?

Have you figured out what makes the game un-fun?

It sounds like you might be lacking a proper sense of progression and pacing.

Your players should gain new abilities (in your game, maybe this could be new weapons that work in different ways), learn how to use those abilities, master the ability, and then make use of their mastery.  Then, they get a new ability and go through the cycle.  That is, you need to figure out what your skill atoms are, and establish loops and arcs.

You want the gameplay to be intrinsically rewarding. That is, playing the game should be fun without any external reward.

You might then also add extrinsic rewards (i.e. things external to the game) such as scores, achievements, extra skins for the characters, etc.

You could improve enemy AI so they can form flocks and have the ability to surround the player and collectibles, and they can block a narrow path to proceed to new areas (currently the player can go anywhere without concentrated resistance against him).

So for the player it becomes much more satisfactory to collect, power up and proceed.

Also this gives the player a better reason to attack enemies at all. (Most games just throw the player into something and he has to kill anything that moves. Without a reason that's not motivating. A bit of story can help here too, e.g. showing how zombies ate his girlfriend.)

You should also show the gun he has, better guns become bigger, and black bullets would be better visible.

Hi, my two cents:

- all you basically can do right now is run around and shoot, and shooting doesn't look fun. Instead of just a ball(?) show a gun on the player, show projectiles and let shooting have more impact. It needs to go !!!BOOOM!!!, things should splash around and be destroyed, a visible and audible reaction to the bullets and explosions. And show the aftermath, let bodies and gore lie around. Check this:

- it doesn't look as if it's hard to evade the enemies. Perhaps add more types of enemies, a few faster ones, a few that explode close to the player, a few that can shoot, the usual stuff. Also let the player move into confined spaces, so that it's actually harder to avoid enemies.

Good luck!

Or consider the opposite approach. Temporarily remove all the bells and whistles, and replace all your graphics with solid-colored squares. This will let you see the core game loop more clearly. If the core game loop isn't fun yet, it's unlikely that adding glitter on top of it will make it fun.

Killing things doesnt seems fun in that video.

I'd add flashy things. Like kill combo messages, or powerups for killing X amount of zombies in less than Y amount of time. With a sort of deep narrator voice and a flashy message on the screen.

The combo thing is something you can keep track off. How many x5 kill combos, x10, x25, x50, and some absurd x100 combo. 

Also the countdown for the next wave? That also could be narrated with some screen feedback like "THREE! TWO! ONE!" That kind of thing.

Zombies dropping stuff like coins or something to buy better gear.

You know, things to keep the intensity going, to keep the player running and killing stuff.

Look at this, it might give you some ideas:

Guys, big thanks for the replies, you helped me out alot, I added blood particles, shaky screen, smooth camera movement, bigger and faster bullets, louder shooting sounds and now it feels a lot more satisfying to kill stuff

Can you share another video? It'd be great to see what you've decided to put in.

Add more need of food or ammo or something so that the player is pressed for finding supplies. They they need to split their attention and surviving becomes more meaningful.

Maybe have periods of calm where you can apply bandage, eat some food before the shit hits the fan again. More variety: calm sections and really hectic ones. Now everything seem like the same thing.