There's absolutely no way that Warners are putting together a writer's room that expensive for just one film. Especially given that they've probably already spent anything from three to five million dollars thus far in development costs.
They're almost certainly kicking around ideas for a 4th and maybe even a 5th Sherlock Holmes film. With Guy Ritchie unlikely to return and Downey likely done with Marvel (in a big, time consuming way anyway) after Infinity Wars it's probable that WB want RDJ to make Sherlock his number one franchise moving forward.
Which is excellent news if you ask me.