As I’ve seen, RocketChat offers an open-source alternative for Slack. I’ve seen a few other commercial competitors for Slack, which offers additional features. Perheps it should be considered to add them to RocketChat.
Such are Flock and Microsoft Teams. For example, I’ve seen they offer different types of app integration, Teams with tabs, Flock to most of the components of the system (they published a YouTube video comparing their product to Slack).