Great execution, I had the same idea a while ago after playing with mitmproxy. My concern would be that people would tentative towards using it as you can tamper with requests on the server which they don't have control of.
Thanks! I figured (most) people were pretty fast and loose with their security these days, so that wasn't my concern. I've found the problem is that it still isn't easy to use, you still need to configure a proxy with credentials and install a root certificate to proxy HTTPS traffic. Lots of vistors to debugproxy today, not that many requests through proxy.