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The thing that turns this from a gimmick into your game is that ammo stops being damage and becomes fuel. Once shooting is the only way to move, running dry is a more interesting failure than dying, so it's worth building the stages around being stranded rather than around being killed.

That hands you obstacles for free. Surfaces that swallow knockback, so you have to arrive with momentum you banked earlier. A long gap with the only pickup at its midpoint, so you commit before you can see the landing. An enemy that only takes damage from one side, which costs you shots to line up, so killing it becomes a movement decision instead of a combat one.

One thing worth testing early: if every weapon pushes you the same distance, there is no decision in the shot. Vary the recoil per weapon and the aiming turns into the game.

My first suggestion is to give unlimited ammo for whatever gun player could wield in this game. This will allow player to express themselves freely without worrying about ammo.

Speaking of gun, my second suggestion is to give various guns for players. At least these 3 guns:

1. Default gun, shoot rate = medium, knockback = medium, damage = medium.

2. Shotgun, shoot rate = low, knockback = high, damage = medium to high.

3. Ingram, shoot rate = high, knockback = low, damage = low.

As for enemies, currently I have two types in my head:

1. Chaser; this enemy moves horizontally but will always face player and try to touch player to damage player.

2. Upshoot; this enemy moves horizontally while shooting upwards periodically. It doesn't care where player is meaning it won't chase player.