Edit: I know my comment sounds like I'm talking about modules more than I'm talking about StaSh commands, but I think I have an idea so just bare with me for now :D
I support this idea. Pythonista is amazing, but when it comes to working out dependency or environment issues sometimes I wanna blow my brains out. I think a great starting point for this package manager would be to use all of the current custom modules that omz has directly integrated, like beautifulsoup, flask, Jedi, jinja2, numpy, paramiko, etc.
There definitely needs to be a more dedicated StaSh solution other than pip. Maybe I'm just stupid and there's a better way, but most the time I start installing stuff from pip I just end up breaking my StaSh install with a ton of incompatible dependencies.