We demand a sequal.

I agree with the many of the comments below and the sentiments the op is expressing. A lot of potential that just didn’t pan out. I’m sure we’ll see mirror universe again at some point, but this would have been better or a six parteer, or better yet, two four parters.

they never did promise we would see those characters
 
are you this new to variant covers?
 

@Archonix  
What’s funny (?) is that the original image was intended as a joke/troll image, but some fans and some haters of the latest arc took it seriously.

Oh gee, the cover designed to illustrate that they’re travelling to an “evil” mirror universe showed mirror counterparts of all the characters for dramatic effect.
 
jesus christ, people. You make drama out of EVERYTHING.

@Sr Pelusa  
I think it was more what they ended up getting instead. If the plot was (to them) awesome, then people wouldn’t be as bugged by the counterparts not appearing. However, since many found the actual plot to be of questionable quality, it just didn’t seem to be worth not having the counterparts in the spotlight.

Ho ho ho. Comic publishers do it all the time n00b:  

Going by this, some of the previous IDW covers would be nothing but a big splotch of red.

@Anonycat
 
Pacing issues and a lack of proper pre-planning. It was an arc that simply had too much going to be condensed into just 4 issues with far too many filler pages than was actually necessary.

@SpikeSpiegel  
The potential held by this arc was good, and shipping Celestia with good Sombra was also an interesting idea.
 
I still can’t guess what went so wrong.

@Anonycat
 
She should really take this opportunity to do proper planning for any future arcs - this one was all over the place, and I actually liked the first issue.

@Dr_Robotpony  
It was destroyed but then Twilight got full access to Starswirl’s secret library and Celestia even keeps a shard of the mirror as a memento, so the possibility of another mirror being built is high.
 
It’s very likely that Cook planned to leave enough loose ends to later write a story involving a return to the mirror world, but after the negative reception of her arc, probably she will not.

Plus there was reversed chrysalis and derpy in the giant stained glass mirror universe windows.

@PonyPon  
I don’t know, I haven’t read the comics.

@Dr_Robotpony  
I thought it was destroyed.

I didn’t read the comic, but so what? Unless the portal was destroyed or something that world can still be visited in future comics.

If you’re going to edit my troll image at least take the 2 extra seconds to open the circle tool for god sakes.

@Background Pony #351D  
Wait Vinyl was there too. I must read it. :)

I have no choice but to disagree with you. There is one page where we see all those characters on the title cover.

@Background Pony #351D  
I think they mean as in more than just a cameo.

WRONG

I giggled at that half-uncheck on Sombra.

they are in #20 page 14!
 

And did Mirror-Angel appear?