Player meta has been to lock every door in the bunker making any new visitors required to bring a lock pick, and hope they can open all the doors.
Some doors are metal which is hard or impossible to melee or shoot open.

The bunker doors with the wheel handle mechanism shouldn't be able to be locked with a lockpick.

Maybe be able to use a crowbar to jam the door for paths that have 2 ways in.

I like the mechanic used to open at the exit, maybe in a future release create a control room with door switches like you'd see at a secure prison.
The operator can override/unlock doors remotely. DayZ doesn't have really any puzzles (do this to do that) unless you play a modded version like Namalsk....

I saw on someone's streams the valves in the bunker, that's a great addition to dayz!!!