I haven't found a multi-protocol IM that supports Slack without loading the Slack app entirely. E.g. Rambox just renders the Slack app.
I used to use Pidgin to connect to several communication networks via a common UI, even with the promise of having a single contact list:
I just realized that Pidgin may have a community-maintained Slack plugin!
https://developer.pidgin.im/wiki/ThirdPartyPlugins
We need a federated communication protocol, similar to email, where we can have real-time communication with people and groups on various networks and using differing software.
If you're allowing your client to be multi-protocol then why is Slack's protocol unsuitable? You can use weechat with the Slack API right now and it works great.
The market for 3rd party applications using Slack is huge.