Full API to write/receive messages so we can create new clients or integrate it into other tools
Slack has a full API, so people can write their own client apps or integrate Slack messaging into existing apps. Would be great to have something similar. Especially combined with giving external users access to Teams.
There is a webhook notification API for other events in the Microsoft Graph API . There is also a REST API  for reading and writing 1:1 chat and group messages/threads in beta. If the chat/group messages and replies could be sent via a subscription using a webhook—or better yet, a WebSockets connection—this would useful for integration.
Igor Baidiuk commented
Current desktop client is just awful, half-baked piece of something which can be barely called software. If you have such difficulties implementing simple chat app, then you can open API for others to implement good client on their own.
Guillaume Meyer commented
Would be useful too to develop migration services from other chat-based tools or ESN...
+1. This is a major blocker for adoption of Teams within our organisation
Toby Beresford commented
+1 - our use case is different - we want to provide an analytics service for our users so they can see how many posts they've done, likes received etc.
Emiliano Parizzi commented
+1 - We need a full API similar to Slack with real time messaging features.
Craig Deubler commented
This is a blocker for us. We need to be able to programmatically DM a user.
This is an important feature that whose absence will limit my organizations ability to fully adopt this product in place of Slack.