Some of this stuff has been hinted at before. Also in theory we’re seeing a culmination of a storyline that started with the Timm/McDuffie era movies if not the TV series. This Flash has rebooted his universe twice from the Timm universe to Flashpoint to the Darkseid War to this current universe and jumped between realities a few times and now we’re here.
I thought the random bum was Constantine, but yeah this is in theory a bunch of movies coming together. Credit to DC for not rushing it and making it a trilogy. My only real hope is we see them do Kingdom Come next.
As someone who watched the Timm/McDuffie era almost religiously, the connection here wasn't nearly as coherent for me but part of that may be the 20-23 year gap and how decoupled the Timm/McDuff verse felt from the Flashpoint/Darkseid War animation era.
At this point, simply making a nice stand alone trilogy (and this Part 2 will be the linchpin for that) should be their focus imo, even if these core JL characters are not from any previous continuity. We should walk away from Part 2 having stronger ties with Supergirl and the Batfamily since those seem to be the focal point of part 2.