While Ben Affleck hasn't confirmed if he will direct the solo Batman movie, it is rumored he is writing the movie with DC Comics co-publisher Geoff Johns (Update: this is confirmed).
Potential details about the new solo Batman movie include:
• Affleck and Johns are said to be adapting the DC Comics Batman storylines "A Death in the Family" and "Under The Hood."
• The villains will be the Red Hood (Jason Todd) and Joker, with the Red Hood being the key player and said to be a priority for DC (à la Marvel's Winter Soldier).
• The plot of the solo Batman movie is stated to be: "Jason Todd has returned from the grave as the Red Hood, working in the shadows to make people think he's Batman, while making Batman look like a criminal. At the same time he leaves clues about his identity for Bruce/Batman to ultimately solve, leading to a confrontation that will involve Batman, Red Hood, and yes, The Joker, who will feature as a primary villain alongside Red Hood. "
• Dick Grayson will help Batman as Nightwing and may be referenced in Batman Vs. Superman (Update: didn't happen).
• There may be a Red Hood cameo in Batman Vs. Superman (update: didn't happen) and/or Suicide Squad.
• In regards to Red Hood casting, it's said WB is looking for a relatively unknown (similar to Ray Fisher getting cast as Cyborg).
• Regarding the look of the Red Hood, it's said to be similar to the comics and the recent Batman: Arkham Knight video game.
• It's speculated the Batman solo film could lead to Red Hood and Nightwing Spinoff movies.