I haven't been following the development of Detective Pikachu too closely, and for good reason; I want to be utterly surprised when I play it for the first time.
That's not to say that I won't catch the occasional clip, and as we enter the roughly one month 'til launch mark, seeing a few familiar faces isn't such a bad thing. Detective Pikachu charges you with "cracking the case of the rampaging Pokemon" in Ryme City, and you'll evidently battle, dance, and befriend a handful of classic Pokemon.
If you're going in blind I would avoid the below video for spoilery appearances, but otherwise it doesn't give too much away!
via destructoid https://www.destructoid.com/detective-pikachu-looks-like-a-wild-ride-491239.phtml