Suggested improvements:

• Move away from Legacy webhooks for Slack integration, e.g. to allow Allow Custom JSON Payloads for Slack Messages
• Allow attachments in message
• Allow multiple recipients in message
• Use the Automation ⚡icon in Slack as the avatar for the Automation for Jira user, and/or Allow custom icons to be sent through slack message actions
• Better mentions in Slack messages - ability to map display name to Slack UserId
• Add the ability to preview a slack notification
• Slack Action: create a new Slack channel, Used for incident management etc.
• Slack trigger: trigger an Automation rule from a Slack message - It'd be interesting to be able to trigger a rule from a Slack message in a particular channel that matches a regex. Might just be better as bot you can chat to though. A4J can automate messages to be sent from Jira to Slack but not Slack to Jira. It can be done but has to be via a 3rd party app, It would be a great extension to the have the ability to have two way communication between Jira and Slack with some slack slash commands similar to https://blog.trello.com/master-trello-app-for-slack-infographic

Better mentions in Slack messages - ability to map display name to Slack UserId -

At the moment you need to know the slack userId in order to send them a direct message.

We should be able to look up a user by display name and then use that value to send the slack message.
Reference - https://api.slack.com/changelog/2017-09-the-one-about-usernames#mapping

Currently it's hard to mention Jira users (like reporter, assignee) in Slack messages since Slack requires the slack user id.

It should be possible to lookup a user's Slack ID on the fly:

• https://api.slack.com/messaging/composing/formatting#mentioning-users
• https://api.slack.com/methods/users.list

Requires some changes to our message editor though (ADF would help), so we can tell when a user is mentioned in a slack message, to look them up.

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They now are pushing "App" with a web-hook as the way to create incoming webhooks.
They have deprecated the old way - https://api.slack.com/custom-integrations/incoming-webhooks

This is the new way - https://api.slack.com/incoming-webhooks