Why do you want to disrupt Slack? Slack is a great product, and it's not winning based on features and functionality alone, they have a great business team as well.

Don't try to compete with a well run team at the peak of their abilities. Go find an underserved market that has tons of money and incompetent incumbents providing services. Slack is successful because this is what they did, but that has sucked a lot of the upside out of the market.