So here's my theory... Which can go both ways but both result in an announcement of paid DLC coming to the game:
1. Some 5-2 days before March 20th they will announce a Nook Inc Island Getaway Package thank you video by Toom Nook for staying with him and he will also show us a sneak peak of all the new DLC content that will be in the game. This include a big update of old furniture from previous games and everything Pocket Camp has, including clothes and more. Also the big thing: Kapp'n is now on the Pier, to take us to a new big new location where we meet all the old important characters from the older games. This will be similar to the city concept in City Folk.
Nontheless we will find characters like Brewster at a tropical cabin pub, and other important villagers like Harriet from Shampoodle, Dr. Shrunk, Lyle, Digby and more in this new sort of sea hub harbror port area. They will all have their unique shop. The DLC will be available to buy on the 20th. This will be the first paid "DLC pack" to keep the game relevant and also to get extra income. Similar to Smash Bros Ultimate fighterd.
2. The exact same thing I said on the first point, but only difference is this new trailer releases on March 20th but the actually content and everything will be purchaseable around late fall to early winter.