If I read the TOS correctly, you are also not allowed to write any kind of chat bot against it. If the current hype for bots remains, bot access might be another thing to sell.

That might be hard to enforce given that the open source client will make creating unofficial bots fairly trivial.

Given that they already have Spotify and giphy integration I'm going to predict Skype-like monetization with sponsored integrations (e.g. Skype has those sponsored animated emoji things)