So far, judging everything we've been seeing, It seems that the concept is a fusion of futuristic/modern and historical elements working as one.
It started when Stray Kids were spotted filming the MV at Mungyeong, a korean city well known for It's traditional elements.
Since then, we've been expecting the group to come with a possible historical concept.
Apparently, Seungmin was spoiling since before filming.
On July 22th, Stray Kids released the trailer for their upcoming comeback, with both the name of the album 'NOEASY' and the title track 'Thunderous' . Usually, their trailer videos work as an intro for their title track's MV, what ended up letting people confused, because everything was pointing out to a historical concept, but in the video we only can see Cyberpunk/Sci-Fi elements.
During the concept photo teasers and the tracklist's release, fans noticed traditional korean elements, mixed with modern ones in outfits and possible instruments they were holding, everything related to the concept of Thunderous.
On August 12th, It was confirmed the song has korean traditional music as an element combined with other sounds/instruments, along with informing about what the song's lyrics is about. Also, how the name of the title track has a significative historical correlaction
Super Junior Eunhyuk also described his impression on the song:
Today, Stray Kids released two concept group images where we can see them using modern-traditional outfits while standing in and Octogonal Pavillion where I believe to be on Mungyeong and another one that seems kind of a cyberpunk background with new modern-traditional outfits.