A quite often underestimated or even forgotten production technique is to take advantage of artificial early reflections which could be added somewhere during the mixing process. Without inserting any fully fledged reverberation at all, applying such techniques allows to dramatically increase stereo width and depth perception as well as a way better instrument localization even in a busy mix. Creating density is not the goal here but the opposite is the name of the game: achieving a clear and intelligible mix.
In a simple case, one can place a short and plain delay (a slap-back echo) on a track and properly place it in the stereo field – maybe on the opposite side of the source but that’s just an example. More sophisticated tap delays could be used to create a sort of room experience and some reverberators are allowing to disable the late reverb diffusion and just to use their early reflection generation. There are no restrictions in general – allowed is what gets the job done in that specific mixing situation.