Here are some detective, mysterious VNs with a somewhat dark setting depending on how you view it.
G-Senjou no Maou ,Sharin no Kuni
These two fit perfectly, they are extremely heavy-story-based VNs that just make each and every heroine lovable in their own unique way (although if you had your eyes set on one specific heroin, you will have to go through the other heroine's chapters first to get to her)
Koihime Musou
This VN is if you are into the Romance of the Three Kingdoms story but all those generals in that era are females! So many heroines to acquire your mind will explode! The gameplay of this VN is somewhat non-challenging and very easy to blitz through so you can imply that they just wanted you to go through the heroines
Symphonic Rain
Since you are playing Little Busters, that implys that you don't only play 18+ games
so you might want to check out Symphonic Rain, a story with a setting about the male character finding a partner for a musical contest or else things will never turn out right. Especially in a city where it rains 24/7. This VN primarily focuses on you playing the piano (mini-game) and requires you to focus on the soundtrack in those specific games... (sadly there is no music outside of the mini-games)
I can recommend more if you want.