Block density and resource cost.

[Headshotter]

>the SMG can go through all bunkers without 5 block thick walls fairly easily
oh god we're off to a great start! such an statement early on means no good
 
>Using the Q and E buttons a player can change the density of the blocks he/she places
and lose the handy quick switch button and a handy E key you can use to bind something else? heck no
 
as for the rest of your idea: why complicate so much? building is already perfect