A PR is quite welcomed, just understand that we currently have a back-log of things that need to be reviewed before adding new stuff (unless it is super compelling; for example if it has an existing analog that was missed in parity with synchronous versions in the standard library or in swift algorithms).
Proposals are also great to write up and flesh out the naming concepts, behavioral structure etc.
I think one thing that might be generally useful is to build up the matrix of behaviors with zip, combineLatest and this; showing a hole in the table of behaviors that this resolves gives a lot clout to its importance.