Slack doesnt need anymore features. Just let me chat and stop making me learn new ways to do things.
How about a native desktop app.
Currently, anyone who wants to write a 3rd-party client has to use the IRC bridge. That doesn't have the same capabilities as Slack's client.
Note this does not use the IRC gateway, but it plugs the slack API into WeeChat, and supports most slack functionality not found with the IRC gateway.
[1]https://github.com/wee-slack/wee-slack/blob/master/README.md
But it's not just chat. With interactive messages, Slack is becoming more like an OS for communications and business processes. And there are seemingly infinite ways for it to evolve to better serve the needs of just about anyone using it to communicate with others.