I will edit this list here every now and then but so far I counted some things that will prevent this game from becoming a “real” big success.
Everything said here is my personal preference and opinion based on playing all previous Xcom titles including the best one TOFTD and EU.
Dont make rounds so long. I waste so much time on waiting for the enemy to do its movement but I waste even more time for the really useless turn of npcs or civilians. Give us the option to either skip it or fast forward it 10xtimes.
Add more color to the game. Old xcom wasnt shy of having really bizarre creative designs. I miss that.
It’s ok how it is now, no doubt, but I can sense the lack of a real nice touch from a good creative artist. I liked the old design which was kinda cool but also very funny in times. I see only cool and a lot of “grey” now. Make cool artist drawings for each research. Don’t just make it a rational things. Let your creative designers get nuts once in a while 
Give us more items we can find when killing stuff. At first it is rewarding to win missions but if you never find something cool, it becomes very repetitive very fast.
Dont make attacks on havens so often. That’s one of the most annoying things we had already in XCOM. In the old xcom even the attacks on ships or city’s were a pain in the ass and took like 2-3 hours sometimes to finish. Dont make those so often on havens aswell as they are really stretching the game and make it so fighting those missions feal like a chore and boring.
Fix the difficulty. I am playing on easy and I tried to do the mission for the 3rd Phoenix Base mission and enemies there oneshot me and I have to shoot them 20 times (not exaggerating) to kill one of them. I am like 5 missions into the game.
Also make the maps more campy or bring back the “fog of war”. I miss the old Terror from the Deep maps where you wanted to explore 2nd floors in buildings or every corner of the map because of the fog. And when you knew some enemies were approaching you positioned your soldiers in a room and let them face the only entrance and then set them to overwatch just to see them eliminate every intruder like lemmings. That was fun. And it didnt matter that maps were repetitive. The fog of war made it like…“Hmm I wonder where this corridor will lead to. Is there a big or a small room after this one…” It gave it more of a “surprise” mechanic even when you already kinda knew the layout.
Improve line of sight. I had some scenarios where I was on the ledge of a door but couldnt shoot at the enemy where the enemy could shoot at me just fine. Also I was at a window and the enemy was clearly in front of me but the map said “nono, you dont see the enemy”
https://gyazo.com/7116642b5c085eff8dad297cbdb91caf
Also, and this might be the most nitpicky of them so far. If I order a soldier to move somewhere let my mouse be free and click the next soldier before the other one finishes his movement. Cause now I have to wait until this guy finishes its movement even when nothing happens while he moved from A to B. You know, how it used to be in the old xcom games aswell.