My major preference for Slack was replacing Skype chats, so there was no loss in terms of open source.

Lots of people complain that it's just a fancy IRC client, but they don't realise how painless it is to get non-tech people to start using it.

But quite clearly there needs to be a fully open sourced alternative.

I think WordPress is a great example of how it should be done. Provide a super simple way to create xxx.slackclone.com in exactly the same number of steps as it takes to create a slack group whilst also providing something that you can download and install yourself.

I think any project would have to be in PHP. Perhaps that sounds stupid but I think again following the WordPress model is a good track. PHP is the most easy way for low-tech people to get their own up and running. It should be as easy as the 5-minute WordPress install to get your own server.

edit: Plus PHP is what Slack is built on and I quite like the irony. The further irony though is that WordPress use Slack now. Perhaps in a similar way that WordPress fought against the React licensing they might replace Slack if there was a good enough clone.

As far as I can tell the aim could be to create as close as possible a clone of slack, ripping off as many features as possible without stepping over a boundary where Slack could try and shut you down.