Your objective: Sneak into a high security facility, plant and arm a bomb, and then escape.
This could probably work as a top down or first person game, and you could handle it in a multitude of ways - stealth, loud, a mixture. You have as much time as you want to break in and plant the bomb, but once you’ve armed the bomb you only have a limited amount of time to escape. Alerted enemies could lock doors or attempt to stop you with combat, anything to keep you from escaping before the bomb goes off.
Wouldn’t need to be a complex game, just have enough elements and variety for a bit of fun. Could possibly incorporate procedural generation if you wanted to add an extra bit of challenge.
Could be as crazy or simple as you wanted, could include roguelike or hacking elements, or even have enemy varieties and different loadouts.