When will there be an Xfinity Connect for outlook 2016?
It's impossible to say that because on the official website, there are mentioned only the available operating systems and versions of Office for which the plug-in is available.
Supported versions:
As you can see, there is nowhere specified about the 2016 version.
Official page: http://customer.xfinity.com/help-and-support/internet/access-outlook-plugin/