Sorry, it's in pre-production not filming. I spoke too soon. But yes, they are expecting that Ian, Tim, Dillahunt, etc will all want to be involved.
**** you so hard if you're wrong. Easily the best TV show in history. Just watched it again recently. Was so good that I even read most of a long essay about the character Wu, of all things.
Garrett Dillahunt has been pretty vocal in the past six months about hearing that the idea of a continuation was picking up traction with HBO. Recently it was rumored that Milch is writing the scripts right now and HBO is getting close to a green light. If Milch can hit their deadline and still feel good about what is on paper HBO is going to cut them a huge check.
Olyphant and McShane would both be coming back, I'm sure Dillahunt would like to play a third character as both Wolcott and McCall are dead. W Earl Brown (Dan) has been getting good work recently, so I see him as desirable for the network. And McRaney as Hearst could be back as well depending on which way the story goes.
The great thing about this cast is that there are quite a few heavy hitters and a couple of them have had more success since Deadwood, but they've all leveled out to a place where they could get the ensemble back together without breaking the bank. HBO is also in a lot better place as far as original programming budgets than it was 10 years ago.
If I had to guess, I'd say it's 75% likely to happen.