A simple chat (slack style) it's enough. Just having the possibility to send messages either single user or team (#channel). But please don't forgive to give an useful appeal to mobile app (worst error of competitors is, have a good app but totally no mobile friendly!) i would expect to have a similar whatsapp or slack app for messaging within the slack mobile app!
Facebook has been under relentless attack since the Cambridge Analytica scandal in early 2018. Broadcasters and news publishers have declared open season on Mark Zuckerberg, Sheryl Sandberg, and other senior executives at the company. And while not quite ubiquitous, #deletefacebook pops up every time there’s a story about data privacy. The EU has fined them, the US is trying to figure out how to regulate them, and the notion that free services should be absolutely free (as opposed to checking a box on a terms and conditions page that allows the free service to use your data as payment) is gaining traction.
For those determined to exit the Facebook ecosystem, the best approach is more likely to be a patchwork of sites and apps that mirror individual features. Messaging is the easiest: apps such as Telegram and Signal offer messaging and group chats, as well as voice calls, with encryption to keep your communications private. Telegram even has a thriving collection of chatbots, similar to Facebook Messenger.
We would much rather prefer to use specialized tools (Google Hangouts Chat, Microsoft Teams, Slack, Atlassian Stride) that focus on being a communication hub and integrate with all the other software we use like erp, crm etc. So a focus on integrating existing tools would be very much preferred. And if you really would like to built a chat platform like e.g. Asana with the chance of being subpar, then please make it optional with the possibility to toggle it off.