The quest to reinvent Lotus Notes continues; also see Take your pick (Slack blog)
"Message menus are drop-down menus that developers can incorporate into their Slack apps. Not everything can be responded to with a couple of buttons, but when you’re dealing with information pulled from a database, a drop-down menu may be best. One example is if you’re connecting Slack with a customer relationship management tool like Salesforce, you could incorporate a Message menu to show relevant accounts to the user. This added feature is geared towards scenarios that require more nuanced decision-making and not a binary response.
There are five different types of menus that developers can use: static menus, which present a set of fixed choices; user menus, which show the members of the Slack team; channel menus, which provide public channels; conversation menus, which list all the channels (public and private) and direct messages; and live menus, which load dynamically based on a server’s response."Slack launches in-message drop-down menus for apps | VentureBeat | Dev | by Ken Yeung