Gab.ai is a platform that is similar to Twitter. You have 300 characters with which to make your point. It has been called the Alt-Right’s social media alternative and although Gab itself doesn’t censor its users, Microsoft has threatened to take them down due to “hate speech.” A lot of folks who got banned, shadowbanned, or censored by Twitter are there.
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!