This is a great idea, but… how many of these emails are you getting? Is it worth the effort to create all this automation? And honestly, in my view it is a bit odd to post an email someone sends to a public forum. I would be annoyed if that happened to me! And so I don’t think this is going to be supported in core Discourse.
If I were you, I’d have these emails delivered to a group inbox monitored by staff (can be volunteers) who can then decide if they want to open a public topic on the issue. If they do, they can then start the topic and invite the user to join the topic and follow up. It’s become easy for staff to invite email addresses and to specify a topic to send the user to the first time they log in.