I've found mitmproxy (https://mitmproxy.org) in reverse mode really useful. I'm a mobile dev and when the backend work is still pending I usually mock the backend requirements by selectively patching or adding endpoint support, it can be used like so:

    mitmproxy --mode reverse:https://my.privateapi.com -s ./path/to/mockscript.py

This allows me to implement most of the UI and server interactions w/o being blocked by server work.