At Anime Expo, the second half of Sword Art Online: Alicization, subtitled War of Underworld, had a teaser video shown. Below, you can find the tweet containing the short teaser:
— アニメ ソードアート・オンライン 公式 (@sao_anime) July 7, 2019
In addition, a blooper reel was also shown at Anime Expo from the English dub of the anime. It was pretty hilarious but some of it contains mature language. Watch at your own discretion!