For the opensource side. https://www.opsi.org may have something of what you're looking at. I tested it out in the past but SCCM fit with what the management was looking to do.
Just to be clear, I'm not as concerned about the deployment side of things (yet). Do how much is opsi and SCCM: Software Center oriented around end users and how much around administration? I know it should be part of the fabric. I'm just trying to modernize some processes that have been enshrined in the halls of familiarity, and this is the first step for us.
Software Center is almost 100% end user oriented. If you are looking to automate this without hands you wouldn't even expose the interface. I think Opsi is the opposite. I don't think, if I recall, it has any real means for the end users to request applications outside of a really bad web interface... even then I may be mistaken.