Google should build slack. Its a travesty how incredibly good their google workspace suite of tools is, and then google chat is what sits between it all. If it wasn't for the fact that google bungled an internal communication tool so badly, slack wouldn't even have to exist.
For the life of me I cannot understand why they after a decade, has let slack and teams become basically a duopoly in this space.
Source: I use google chat everyday, so its not just a "UI looks ugly thing". Literally nothing you think should work works. Ex: inviting outside collaborators to a shared channel, converting a private DM group into a channel, having public channels for community & private channels for internal work. Goes on and on.
In my experience building a couple ChatGPT apps and, through colleagues, working with OpenAI folks, I'm not sure they should be building anything in their current state. Seems quite disorganized over there.
There's already Zulip, Mattermost, and many others. Building a chat application should be considered a tar pit problem IMO. A lot of success relies on network effects and familiarity, and the product looks deceptively simple.
It's unlikely you can build one that is better than Slack without years of investment. Even if you do, it's still an uphill battle.