as i have switched to UT a few weeks back and use Slack quite often (personally and for business) i was also thinking about creating a Slack client. The main problem was the way slack is constructed its not directly possible to have a fully standalone client.
I was following the approach of an Slack App and quickly realized that those apps can only communicate within the workspace they have been created for.
I decided to reach out to Slacks tech support, the support was quite informative and as they understood what i wanted to do they told me that its not possible to create a fully fletched standalone client with the current APIs they provide. As the API seems to be centered around the slack apps. Which according to the API documentation does not have a single endpoint for switching workspaces.
I was able to create a prototype to have this whole OAuth2 token generation part and the Connection to the RTM/WebAPI ready but was bound to the workspace my slack app was created for.
On the other hand the app department of slack was quite interested in providing an UbuntuTouch client. They told me they will discuss this and have put this on their feature requests.
I mean possibly there could be a way to have this running without slack or the endpoints. But i believe this involves heavy reverse engineering and network sniffing in order to get the correct communication to be able to get this working as a standalone client.
The support team told me they would get in touch again which should be this week to have me informed about possible progress. But i would not count on this.