In the trailer, it shows off many of DC heroes, the ones seen on the CW so far including Black Lightning! Then further into the teaser they use a lot of scenes from various shows but mainly Supergirl, They reused the scene of Supergirl talking about how her cousin worked with a vigilante with crazy weapons and at the end of the trailer, it showed another scene of Supergirl saying and quote "I thought masks were only big in that other city! You're gonna tell me who you are?"
So that line was from Season 3 when Supergirl first encountered Reign and when Supergirl said "I though masks were big in that other city" That was referencing Gotham.
So does this teaser trailer hint at Batwoman actually existing on Earth 38 instead of Earth 1?