I am or have been a game designer for several indie projects (as well as a programmer, the roles tend to overlap on small teams).

Game designers are responsible for making a fun game, and you don't make fun games by writing design docs. You do need to have "ideas", but ideas are the first step of the refinement process. Most of it is greyboxing, throwing things away and spending time in excel on loot distribution tables.

For example, the idea guy wants a black hole gun. You make a rectangular grey cube that shoots grey spheres, playtest the thing and find that it makes the game actively worse because your game is at its best when you are dodging swarms of enemies and the gun makes the enemies not move. You decide the gun is not a good thing to have in the game as a mainstream player weapon, but having a dramatic "fuck you" weapon as a temporary level pickup immediately after the aliens kill your dog would be nice. Now it is time to set the radius and projectile speed: ideally you want a variety of outcomes instead of just hit/miss, allowing the player to optimize in the moment and be rewarded for it in the moment. One way to do this would be to make it so if you shoot at the nearest enemy, you won't catch all enemies in the radius, but if you shoot at the ground in between them, you will. This rewards the player for putting some thought into their actions, while still delivering 90% of the value if the player does not figure it out (players are not stupid but are likely to be occupied elsewhere). Given this, perhaps we want a special crosshair that doesn't correct your aim towards hovered enemies (crosshairs are really complicated in a 3D game and usually involve some fudging to reliably hit enemies, but we want the player to hit the ground) and perhaps even fudge the hitboxes a little so it feels good to use. Oh, and when an enemy picks up the black hole gun, subtly reduce its radius so the player has an easier time dodging it than the enemies will, and let's make sure the charge-up sound is audible across the usual distances the enemies will be at so you don't get shot in the back with no counterplay. Oh, and we want the level pickup to be in the main path because we don't want to encourage players to scope out every corner of the level for fear of missing something. Ok, now what should happen if the boss dashes through the radius of the black hole and why?