Premise: Jumanji transforms itself into an app. It becomes a huge bestseller, and everyone downloads it. It starts out completely normal without anyone being sucked in. The kids from the last movie learn about the app, and try to warn people about what it could do. Nobody listens, and it's too late, and the all the people with the app get sucked into Jumanji. Therefore, it's up to the kids from the last movie to turn back into their avatars and save the day.
There's two ways you can do this: The first, more likely way, would be to have the people from the real world turn into their own unique avatars. This allows more starpower like Channing Tatum or Emma Stone to join the party.
The other, and the more fun one, would be having the real world people turn into one of the four avatars from the last movie. Having one Johnson, Hart, and Black in a movie was already great. But having hundreds, thousands, millions of Johnsons, Harts, and Blacks running around the jungle, each with unique personalities and tropes, will be an easy sell to put in all of the advertising.
Of course, if Johnson's too busy with filming other movies and preparing his presidential campaign, they'll probably go for the first option, and have the kid who turned into Johnson have some subplot with the remaining denizens on Earth.