As it stands, if you're on Discord for any amount of time you're inevitably going to close a DM. Be it that you need to trim down the amount of DM's you have open, a reluctance to continue talking to someone, or whatever other reason.
Unfortunately, we're human beings and we tend to make mistakes. You're absentmindedly closing DM's and then... Whoops, misclick. Now that DM with critical information is gone. And, due to how life affects people, that DM might belong to someone who doesn't use discord anymore and if they're not on your friends list or in a server with you... You're out of luck.
Thus, I'd like to propose the ability for users to re-open any DM they've previously closed. If they're stored somewhere, why not let people re-open them again in the event of a misclick? It would certainly help out in the long run for users who like to keep things tidy but are capable of making a mistake here or there.