I too am in the camp where I'd like to see the 135mm prime get some love first.
I agree that they'd probably make an amazing 135 (I'm a huge ART-Line fan), but I'm pretty sure they can't compete with Canon's Price Point of about $1k for their VERY good 135 f2. If they come out with a lens faster than f2 or with OS, I'd be interested. However, I doubt there's really any money in it for them with Canon's stellar lens.
I'm pretty sure they could really cut into Canon's market share with a good new 85 ART since Canon's equivalent (and amazing) lens is $2K+. I'd been waiting for Sigma's 85, but couldn't wait any longer and pulled the trigger on the Canon. I'll be selling it if Sigma's 85 is as good as their other ART lenses.
I think you've gotta follow the money here, and Sigma's gonna make more with an 85 coming first (imho).