I love the idea of a Vacation responder plugin along the lines you’d see in gmail. It would help in communities like mine that use discourse in an organization.
When setting it on your preferences, you could specify groups who see the message, e.g. “Only send a response to people in these groups”.
When you mention someone who has a vacation responder set, you’d see the message pop up right away, akin to the blue notice that pops up when you mention someone who is not allowed to see it - e.g. like “You mentioned @foobar but they won’t be notified because they are unable to see this personal message. You will need to invite them to this PM.” you’d see “You mentioned @foobar but they won’t respond until they return on xyz date. Here’s their message: xxx”
“offline” Avatar flair would be helpful too, visible in discussions, as well as the date return indicated on user profile, user card popup.