The phrase "mysterious trailer" is an idiomatic expression that refers to a preview or teaser for something that is not fully understood. Full definition
Kojima and his crew recently debuted a new mysterious trailer for Death Stranding at The Game Awards 2017. (wccftech.com)
Death Stranding is still a long way away from completion, probably due at tail end of 2018 at the earliest, more than likely 2019, but Kojima - san has been very forthright in teasing us with mysterious trailers. (fingerguns.net)
Kojima and his crew recently debuted a new mysterious trailer for Death Stranding at The Game Awards 2017. (wccftech.com)