More options are coming to the Message Compose experience!
Check out these mock ups for what this could look like.
When composing a new message, you'll be able to:
Select a label and use it to create your recipient list
Select a conversation assignee and use that as a filter to create your recipient list
Multi-select all from a conversation list to build your recipient list, not just from the first page of results
Import a CSV file with phone numbers directly when composing a new message
Drag & Drop file support
Insert Google Sheets link (rather than file upload)