Here is a quick list of the things I miss most in this game. I think most of them are a must, and no matter what the effort should be implemented. Please add your comments or features that you miss.
1)
Saving in missions - it's obvious - and for those "unique" moments when you did a perfect mission until turn #30+, and the last alien killed your best soldier in the squad with a knife...
2)
Destructable terrain: walls, floors, cars, shelves, furniture, everything that isn't made of stone or xenotronium (whatever). In X-Com one could blast a hole in a wall with a rocket launcher and get in (I used this tactic with alien ships... I know, the guys from Area 51 cursed me all the time - oh Commander, can't you retrieve an UFO in one piece?) :lol:
2.1)
Shoot through walls. In X-Com one could fire a weapon through some walls - in barns, houses, even in some alien facilities. It's natural to want your assault rifle burst to pass through a country wooden fence or through the walls of a Japonese hut!
3)
Doors - you never knew what lied behind a closed door - was it an alien a turn ago, or a dumb civilian?
I would also like the posibility to lock/jam a door, and of course, blowing it up.
4)
The screams - this should be 2nd or 1st on my list. The old sounds were poor, but very effective and set the mood towards scary. That's why were so great after all! I also loved the sound of footsteps and opening/closing doors.
5) The
"shoot behind the corner" (guided) rocket launcher 
- yeah... you could shoot with that rocket launcher through a maze if you wanted, and it was cool. Maybe it won't be considered, because doesn't fit the setting or it's unrealistic, but it was cool, and had to be remembered.
6) The
cargo ship map - it had countless rooms, and a huge hold - it was impressive.
7)
Panicated aliens, droping their weapon and running away
Aliens going bezerk...
X) The Lobsterman
... yea` it was the coolest alien... I had to mention it, even though it won't be in the game
