Or just plug it into a running campaign: A wizard was using another smaller plane of existence as this, but they have somehow become unable to close it again and the party need to venture inside to fix it so he may safely close it again!
Now I want to know wether the rules allow you to stand perfectly inside your mirror image (don’t have a 5th ed./any other D&D ed. rulebook to check myself, sorry)… Because if it does (without going all wavy or otherwise distorting) and it casts a reflection this could be a fun “find the hidden vampire in this banquet hall with mirrors” scenario!
On first glance I was actually reminded of FesZ Electronics!
Oh but the fireworks of Ericas “broader detect magic” became so popular that she actually added back all the spark colors for all the moral edge cases!
We now have novice wizards playing around with exactly how angry they need to be and how gaudy their robes need to be to get the different signals triggered…