Another lesson about the dangers of relying on a separate team to maintain core part of your software. Not that using dependencies is a bad thing, but one should still be aware of what tradeoff they are making.
In terms of a new backend, there's uber's pyro [1] that's based on pytorch, and then there's Edward [2] that's based on tensorflow (and I think recently got integrated into tensorflow repo). Would be interesting to see pyMC adopt MXNet as its backend. I'm sure Amazon will become a heavy user and might sponsor the development. Or at least the authors will be able to get jobs there without a whiteboarding interview phase.
[1] https://github.com/uber/pyro
[2] http://edwardlib.org/