According to Nokishita, the brand new Tamron 100-400mm f/4.5-5.6 Di VC USD that Tamron announced was under development last month, will apparently be available or preorder on October 26, 2017.
We don't know pricing yet, but I suspect it will come in around the $799 price tag that the Sigma's 100-400 f/4.5-5.6 DG OS comes in at.