I think the open source approach is probably the most likely at this point. To become a meaningful disruptor in the space, you need a product that does somethin

I think the open source approach is probably the most likely at this point. To become a meaningful disruptor in the space, you need a product that does something much better (or at least start of with parity features but free). If you can scale, an open source project with the same features and reliability would be a starting point. Slack also has great customer acquisition/retention metrics so this new product would have to work hard to convince a lot of people to learn a new product for it to be worth it.