According to Variety, they're in talks to take on Bram Stoker's Dracula with Sue Vertue's Hartswood Films producing. The series will follow a similar format as their BBC detective drama, with each season comprised of a limited number of feature-length episodes.
As of now, it's unclear whether it will be adapted to a modern setting and if Cumberbatch will (hopefully) be attached. What we do know is that production has not begun on the new series and that Gatiss and Moffat have no current plans for new Sherlock episodes.
Both are busy with side projects: Moffat is writing the new Jekyll film reportedly starring Chris Evans and Gatiss is attached to several projects, including the miniseries Gunpowder with Kit Harington and Liv Tyler.