Arexx ports for scripting IPC / automation, the standardized installer (especially the "Pretend to install"), and datatypes (which sort of exist in Windows at MacOS, but aren't exactly the same thing either) are the three features I'd love to see in current OS's.
All three would need buy-in from application developers, and since using them would tightly-couple applications to the host platform I can't imagine that any developers would take advantage of them anyway.
We've ended up in this world where a vaguely POSIX-flavored feature set is about the best we can hope for if a developer doesn't want to go "all-in" on a given platform.
My favourite is FrexxEd (co-authored by the author of curl) - a text editor - that exposed all open buffers as files, so you could run script directly on the contents of the open buffer.
All three would need buy-in from application developers, and since using them would tightly-couple applications to the host platform I can't imagine that any developers would take advantage of them anyway.
We've ended up in this world where a vaguely POSIX-flavored feature set is about the best we can hope for if a developer doesn't want to go "all-in" on a given platform.