No Romulans and no Cardassians... (most likely). Who am I supposed to root for without the presence of my favourite quasi-fascist totalitarians? Will the Klingons fill in that role? If they're written in a similar way to the Original Series I can see that happening. But I fear they will still be the dumb (but lovable) honour bound buffons that we know from the other series. Some people mentioned the Sheliak, but they're too alien. The same with the Tholians. Perhaps an entirely new species (but I can already hear the canon keepers screaming)? How to solve this conundrum?
Then why bother placing the show in the prime universe? Just do a reboot! I wouldn't mind at all. Specially since it would increase the likelyhood of Romulan and Cardassian participation and the presence of their superior form of government and morals.
Because saying the words "Prime Universe" is good PR. Done! Henceforth we shall speak of the Discovery Universe.
First contact with the Cardassians was prior to the 2250's according to the new movies, I don't see that being different in the prime timeline. There has to have been enough time for the long history implied by TNG to have happened. Discovery will be exploring areas away from the Enterprise 1701 and her missions, so she could go closer to Cardassian space, and let us see the earliest days of the um, 'issues' the two had back in the day.
There is only one constant in all trek universes- there will be always some fans complaining about something
They don't all have the forehead brow. The Tal Shiar probably riddled the Federation, posing as Vulcans before and after kirk found out what they looked like.
How did Tobin Dax know about Fermat's theorem, let alone about Wiles' demonstration, if the Trill had not met the humans before 2100? And while we're on the subject how come Picard didn't know about Wiles' demonstration?
Every Trek spin-off has introduced entirely new species...so what would be the problem? I don't get the question. Tobin Dax lived after 2100, so why couldn't he know about it?
Sulu brings up the possibility of spies in "Balance of Terror", and I think that the Romulan ship looking similar to current Starfleet reinforces that possibility.
The Cardies are known to Starfleet in the TOS time period (q.v. the 2009 movie name-dropping them.) People can say, yes, alternate timeline, blah-blah-de-blah. But the truth is there's absolutely no reason the Cardassians of this era couldn't be foes for the Federation during the Prime universe period covered by Discovery. It wouldn't contradict anything.
Where did they #### off to at the end of the prequel just before the next series that was made 50 years ago starts? Remember the Clone Wars cartoon? Anikan had a cute animated teenage girl as his padiwan between Attack of the Clones and Revenge of the Sith, who had not ever been mentioned in revenge of the Sith, because she wasn't invented yet. The fan consensus was that her inescapable death was going to be deplorable.
Will this new series really need recurring antagonists? Remember in "Amok Time" the Enterprise was ordered by Starfleet to visit Altair Six: "Altair Six is no ordinary matter. That area is just putting itself together after a long interplanetary conflict. This inauguration will stabilize the entire Altair system. Our appearance there is a demonstration of friendship and strength which will cause ripples clear to the Klingon Empire." (Admiral Komack's dialog) Ten years in the past would mean this system was still in conflict; there are some ripe story possibilities, no?