One of the biggest bafflements of our users is that when they add an event and later come to the conclusion it should have been a recurring one, they cannot change that.
To add the option for this shouldn't be too tedious to implement in the plugin core. The only issues I thought of so far that add complexity to the implementation are comments and bookings. As modifying a recurring event would delete all of them, changing a regular event to a recurring one could just do the same.
So it'd be a case of adding the UI in the template, which would be mostly reusing the when-with-recurring.php and the existing jQuery scripts, and possibly a back-end piece as well.
Our team is not familiar enough with EM's back-end side so we won't be attempting this any time soon, but it might come into picture later on. As this is a feature I'd imagine most anyone who uses recurring events would appreciate, I found this worth requesting in hopes this'd be implemented in the plugin core within, perhaps, the next two months.