Down the pike.
This is one of those frequently misused phrases. When something is on its way or soon to happen it is correct to say that it is coming down the pike.
In coming down the pike, the noun pike is short for turnpike, which is a broad road, sometimes a toll road. This usage of pike originated in the U.S. in the early 19th century, and the earliest known instances of coming down the pike appeared around 1900.
I remember hearing that down the pike had something to do with the medieval pole weapon, but apparently that is not the case. I will just add that to the ever-growing list of things that I thought I knew but didn’t.
As usual in English – and other languages, I assume – when people regularly misuse a phrase the incorrect version becomes common enough that is accepted as valid. That seems to be the case with down the pipe which has become pretty common. However, unless you’re a plumber or a pipe-fitter, I think it’s best to use the original phrase as it was intended.