One of the odd elements of Discovery is how unlike most modern dramas, it's got a very, very small cast. I'm not just talking about the six (well, now five I guess) main cast members, along with the glorified extras which make up the bridge crew. I'm talking about the people who are not even stationed on Discovery, but we keep running into again and again and again.
In the first act, for example, there were only essentially four Klingons - T'Kumva, Voq, L'Rell, and Kol. After these four were introduced by the end of the second episode, the show never introduced an additional Klingon again which was anything more than an extra. With T'Kumva and Kol dead, and Voq/Ash and L'Rell onboard the Discovery, the only free Klingon left that even has a name is Dennas (the woman who wears the jewelry veil).
In terms of non-Klingon guest characters in Act 1, besides extras and those on the Discovery, virtually all the show has is Admiral Cornwell, Mudd, and Sarek. Okay, Amanda Grayson appeared once, and some other characters like Stella, the convicts who try to beat up Burnham in the third episode, and rando admirals make brief appearances, but they can barely be called characters.
So, we go to the mirror universe. Obviously the shtick is seeing mirror versions of characters from the prime universe, so it's not that surprising to see mirror versions of Georgiou, Sarek, Voq, Stamets, Landry, Detmer, Owosekun, Connor, etc. It stretches credulity, but so does every single effing thing in the mirrorverse, so I was willing to let it slide.
But in the trailer for next week, what do we see? We see upon the return to the Prime Universe that Cornwell, Sarek transport onboard - seemingly with the PU versions of the MU rebels which accompanied Voq! Presuming that Mudd can't get away from Stella's tender TLC, this means that literally the only Prime Universe characters which were given more than two lines of dialogue suddenly appear - together - as the first contact Discovery has with the Federation.
Okay, I know this isn't procedural Trek any longer - that it's serialized. And I realize the seasons are a lot shorter these days. But the need to shoehorn the same rotating group of five guest characters into the plot is getting ridiculous, considering how huge the Federation actually is supposed to be. I'd much rather some new characters get introduced at this point - characters we can learn more about from the second season onward. It also feels like it hurts the show greatly, insofar as the "worldbuilding" aspect of this Trek has been very lacking. Instead of feeling like I'm watching a real world somewhere, I feel like I'm watching a stage play where a small cast has to keep filling up the arc.