-By deleting the glass block, it should shoot a bullet. The glass would respawn immediately after the server recognizes that you shot it. Switching weapons would require a text command, like /laser, /gun /rocket, etc. This would allow for many more gun types.
-Because of the very small amount of angles possible, a fair amount of auto aim would be implemented.
-When player health reaches 0, the player could be teleported to a death area, and wait for the next round, or become a ghost who watches the gameplay.
-Flowers could represent bullet shots by changing the texture.
-For the rocket, tnt or bomb, an explosion should be generated in blocks by the server. Anyone caught in the radius of the explosion would have damage done to them.
-Health would be shown either by text, cons being local chat flooding, or by 10 far off blocks, white and red, moved by the server to be at the bottom of your screen.
-An enemies health would be shown on their name above their head by changing the color of the name tag, white at beginning, progressing to red. (not sure if this is possible in real time)
Those are my ideas, smooshed in this post. Sorry if they confuse you.
We need minimal chunk updates so the thing should be far away. Health should show every 10 seconds or when it changes.
I don't know nothin' about mods, but as long as you can get an fps game working in minecraft, I'm all for it! I've always wanted to use some of my minecraft buildings as fps arenas, and I guess this would be a great way to do it!