I'm sure every player at some point in time has encountered a moment in their game when a single zombie strolls up to their base door(s) (isn't killed by a lookout), and winds up breaking their way into the base (it only takes about 10-12 seconds).
My idea would include a new build spot option on all base doors in the base screen for upgrading. This would allow players to use resources and supplies to increase the durability of homesite doors. Upon taking damage, the doors would be repairable incrementally with parts, just as a weapon would. Should the door be broken open, the full repair cost would be 50% the parts cost of the upgrade). Each door has its own individual upgrade and repair cost; both of which must be done through the base screen.
The upgrading would include 3 stages of improvement: Boarded-up, Reinforced, and Zed-Proof. (Upgraded doors would also visually display the improvements)
Boarded-up: A few extra 2x4's and some rusty nails (1x material and 10 parts) *Takes a little longer for zombies to get in; no noticeable difference for ferals or juggernauts.
Reinforced: Scrap metal, some rebar, and some bolts (3x materials, a light vehicle upgrade kit, [for salvage], and 50 parts) **Takes zombies considerably longer to get in. Takes a little longer for ferals to get in and can take an extra hit or two from a juggernaut.
Zed-Proof: 3/4 inch-thick, solid steel plating, some heavy-duty industrial bolts, and some spikes for good measure (5x materials, a heavy vehicle upgrade kit, and 150 parts) ***Takes significantly longer for zombies to get in; ferals struggle to budge it, and juggernauts have to box with it for a short while.
Again the doors require these costs just to upgrade them; repairs will only cost parts, and a full repair from broken will be 50% the original parts cost.