In retrospect, the only thing surprising about yesterday's outta nowhere online drop of the second Avengers: Endgame trailer was that there was no announcement or trailer teaser the day before.
With no warning and no fanfare, Walt Disney debuted the second (and presumably final) theatrical trailer for the 22nd MCU movie. Like the first trailer, it was intentionally cryptic and offered little concrete information in terms of how the movie would play out.
If Marvel and Disney are able to maintain this level of mystery until late April, then they will have essentially turned Avengers: Endgame into the series finale of Lost. We'll walk into theaters knowing that it's the end of the line and knowing that a specific threat needs to be dealt with but otherwise having no clue how the story will wrap itself up.