Nintendo has kept its cards close to its chest in regards to the contents of the E3 Direct, but at least one major announcement has been revealed, as Masahiro Sakurai has confirmed that the next DLC character for Super Smash Bros. Ultimate will be revealed at E3.
Nintendo's E3 presence kicked off with a series of tournaments for Splatoon 2, Super Mario Maker 2, and Super Smash Bros. Ultimate on June 8th. The Super Smash Bros. Ultimate tournament was won by Team Japan, and they received a special message upon achieving victory.
Masahiro Sakurai had recorded a special video message that was shown during the Super Smash Bros. Ultimate tournament. Sakurai congratulated the winners and confirmed that his team was working hard on the next DLC character for the game. Sakurai then confirmed that the identity of the character would be revealed during Nintendo's E3 Direct (thanks, GameXplain).
The announcement of a new Super Smash Bros. Ultimate character might have been the safest bet anyone could make about E3 2019, as there are still four more characters to come, and no other reveals have been made since the release of Joker from Persona 5 in April. There have been numerous rumors concerning the identity of the 3rd DLC character, with a Dragon Quest representative being the most likely choice, but no one will know for sure until the announcement is made.
It's also possible that multiple DLC characters could be announced, with one that is coming soon receiving the bulk of the spotlight, and another being teased for a release later this year.
It's likely that Super Smash Bros. Ultimate won't take up too much time during Nintendo's E3 Direct, as it was the focus of E3 2018, to the point where Nintendo's stock dropped due to a lack of other announcements. It's possible that Super Smash Bros. Ultimate will receive a few minutes during the Direct to announce a character and maybe some new (or returning) features that will be coming in a later update for the game.
Nintendo's E3 Direct will be held on Tuesday, June 11, 2019.