I had an idea that could be useful to make your dwellers safe when your base get's attacked by Death Claws or when fire spreads in your vault.

35 DWELLERS - UNLOCK SAFE ROOM

The safe room would be a room where the children,pregnant woman and low health dwellers could run to prevent death.

It could have a door with health bar(like the vault door)giving time to you to heal the dwellers.

50 DWELLERS- SENTRY GUN: The sentry gun could be a machinegun on top of vault door and with an dweller using it.

The sentry will shoot on raiders or Death Claws when they are rushing to the vault door,making them have less health when inside the base,making easier for the dwellers kill it.

It could be upgraded for increased damage.