The 80's cartoon, maybe. Problem there is that the rights to the original characters created for the cartoon are owned by Disney, while the rights to the name Dungeons & Dragons (as well as the game itself) are owned by Hasbro. And this movie is being made by Warner Brothers, not Disney. So unless this is going to be a WB/Disney team up, and no information has been released to suggest that it will be, it won't be based on the cartoon. D&D.i suppose WB could buy the rights from Disney, but again we haven't heard anything to indicate that to be the case. At least not yet.
As for your suggestion that the movie will be about kids playing D&D, and the events in the movie will be us watching the story unfold in their collective imaginations? Marvel Studios actually rejected that idea for the cartoon back in the 1980's, because the audience wouldn't get invested in the characters if they didn't feel that the characters were in real jeopardy. That's why the cartoon is about six kids trapped in a magical world of Dungeons and Dragons rather than six kids sitting around playing D&D. So I doubt that they will go that route for the same reason.
And WB has already said that the movie will be set in The Forgotten Realms (there's a link to the article above). Which means, unless they scrap that plan in the future, a live action adaptation of the cartoon is out of the question.