I bet that there'll be chance that we'll see shiny Ghastly somewhere around halloween, although it doesn't need to happen. Let us know about when we would most likely see new shiny!
I think shiny unown should spawn around Halloween
Release shiny pokemon in each festival or season
I think that's how Niantic are planning it, but then it would take about 60 years to get all the shinies.
RIP Pokemon GO AD2076 (2016~2076)
I'd like to think I would have caught at least one by then