I think I'd rather see that as a spin-off than anything else. Unless it's Jack Harkness meeting Robin and Marion.
The golden arrow conclusion was freakingly dumb. However, I'll give it a grudging pass.
Will be very surprised if Missy does not have some tie to that Church. It was the first thing I thought of after watching the season opener.
I did love the bit where The Doctor pulled out the arrow and it actually showed the Tardis healing itself. Don't think they've ever actually showed that happening before.