Let's take the idea of a mirror that allows what it sees to come to life--the reflection steps out of the mirror and does battle. You can tell them apart because the evil duplicate is reversed left-to-right (well, technically, mirrors reverse front-to-back, but it looks like left-to-right). (I suppose if you draw a beard on the mirror, the evil duplicates have beards, but....nah, too difficult to manage.)
But what about funhouse mirrors? Could you bend and twist the mirror to make some kind of funhouse copy? Perhaps the mirror creates strong or heavy versions of people, imbuing them with strength or great girth, and creating different kinds of foes. This might be an encounter by itself: the hero is attacked by these funhouse copies, even though the original has no idea of it. It just gets worse when the hero has to go through the funhouse, and copies of himself or herself are created.
What if the mirror cracks? What happens then? A mirror that's a portal becomes two smaller portals, but one that creates copies might create smaller twins or not create twins, or create terribly flawed twins (the bizarro copy?).
From a mechanical standpoint, I find the biggest problem with evil mirrors is that they create too many copies. If you don't have a mechanism in place to limit the number of copies, you can have to deal with one or two dozen villains. (This is one problem with the house of mirrors concept.) But if you have something in place...the mirror has to be hit by lightning or a magic phrase...then it's a fun idea.
Or if there's some kind of time delay. Perhaps the mirror replicates the last person who is entirely reflected in it, but the mirror has to go in the dark for some time afterward to prevent "seeing" something else. Suddenly it sounds like a kind of seance or medium scam: "Gaze into the mirror of Erised and see... Oh, sorry, got to cover it up again."
What about other mirrors around? Dressing mirrors, mirrors in clothes shops, hand mirrors, mirrors in beauty salons, hair salons, barber shops, dentist or doctor office, surgical theatres, little round mirrors used in stores to catch shoplifters, vehicle mirrors (including bus mirrors). What could you do with "Objects in mirror are closer than they appear"? Are the copies smaller? Are they superfast? Do they have growth of some kind so they can suddenly be much closer?
Presumably they've done all of this stuff with Mirror Master, but I haven't seen any Mirror Master stories (though I did see the Flash and Substance episode of JLU).
1. Possibly fruitful typo: "Hearse of Mirrors" My first thought is, evil copies of dead people. They entice living relatives into...something. Signing things away is the boring possibility.