You will have better luck with a different question.

Don't ask 'How do I disrupt Slack?', or more generally 'How do I disrupt Y?'.

Also don't ask 'I want to build Slack for X, what X is good?', or more generally 'I want to build Y for X, what X is good for that Y?'.

Instead observe your environment. Get out and walk around. Talk to people you know in domains you have personal experience in, or are very familiar with. Lots of people - people that are like you, and people that aren't. Find out what their pain points are, and what they'd pay to get rid of those pain points. If you want to focus on chat, find out what their pain points are in communication (the broader area).

Then execute to solve those pain points and make the world a better place. Iterate your execution to solve one pain point (the one the most people say they have/would pay for) and get your first customers. Then solve additional related pain points in successive iterations.