You could start with simple wooden structures that you upgrade and could change the layout/theme just like in the first, there could be added defensive placements and dedicated guard jobs too. I’m imagining with this scenario, attacks would come from both sides of the buildable area; you could make turrets at some point to replace the human guards, and there could be a verticality aspect to it, like building turrets and other things on higher levels and it gives an accuracy/damage bonus.

I’d also like the ability to send out more scouts and mission parties. With more locations to visit, and more enemy types like the Brotherhood or Enclave. I don’t remember if you could in Shelter, but it’d be cool to see other vaults you could visit on missions, either functioning or ruined.