I wholeheartedly recommend the King's Indian Defense.
I have played the KID for about a year and it has greatly helped me understand common structures better. It greatly helped me understand how to play structures with a closed center.
The KID can be a little hard to learn but once you learn the basic ideas it is a strong opening and you don't need to learn a ton of theory.
check out also...
Chigorin Defense
https://www.youtube.com/results?search_query=chigorin+defense
Ragozin Defense/System
https://www.youtube.com/results?search_query=ragozin+defense+chess
be sure to check out 'Hanging Pawns' channel on these defenses.