I feel like the maps theme should not be your focus when you start to build a map, allow the idea for the theme and location develop as you build the layouts, this makes sure that the entire style, when it does come into use fits in with the layout, and you do not waste time early on trying to style a map with no gameplay.
In terms of ideas however you could try the following, these come to mind when I think of maps, in cs:go
1. Something Japanese, construction styles of Japanese buildings are always nice to look at and to your surprise, they don't look bad in hammer either, its a theme I have been working with for a while now.
2. underground location, make it special, maybe its a sewer system for a hostage rescue mission or maybe something brilliant such as the Paris catacombs! where the terrorists must cave in a area of it to take down a structure on the surface, the possibilities are endless.
3.something epic. maybe a government hideout in the mountains with the towns visible through the snow bellow. or even go rpg style and have a fort or castle fight.
4.orange and grey map. because styling just takes too long.