They are still in the script stages, so casting for The Sandman won’t happen until next year or even the year after - depending which release date the movie gets.
I personally think that Joe wouldn’t have gone through the trouble of getting this movie off the ground, if he wasn’t involved in an additional capacity other than producer. (And his involvement is pretty much the reason WB is giving the adaption another go). He will either direct the movie or play a part or both. Whether it’s the titular character remains to be seen. I like Cumberbatch and he definitely fits the role look-wise. But I’m also absolutely convinced Joe can pull Morpheus off.
If they succeed in adapting the sprawling source material in an effective script, this movie has the potential to bring a very different flavor to the superhero genre (or graphic novel adaption). I’m very excited for it.