Just wanna share a few ideas I have. In case if anyone wants to create.

1) NeoTokyo inspired level

Imagine like, Halo ODST New Mombosa level. You and your marines are in an empty city, with dark neon lights, and you have investigate each area, some areas has a PDA to find out what happened to the city, including leftover weapons and items.

Sometimes you even found out some audio recordings that the people in the city has left over.

2) Trench Warfare inspired level

I got this idea after seeing the intro for StarCraft Brood War. Imagine a level in the middle of the battlefield... you and your Marines have to fight through the trenches, with swarms of aliens coming.

At the some point in the level your Commander says that there's a psi emitter tower that you need to destroy because it's causing aliens to come, meanwhile convoy carrying a Nuclear Device has been attacked by Aliens, and your squad has to escort it to the the location.

https://youtu.be/NJZa0OclrMo

I hope this idea helps :)