Actually, as far as the ending goes, one thing that I really hated about XCom 1's ending was the mars mission.
You got one -- only one -- chance. If your first base assault on mars failed, you got "Game over". Not back to your base, and try again later.
And please, please, make sure that Mar's interior walls are strong enough that a blaster bomb can't destroy it. A setup where you walk down a long hallway, into an exposed area, where mind-controlling aliens are going to make your guys fight each other is a great idea. But blowing a hole in the roof of that hallway, sending one person up to look around (and "light up" the room), and sending another blast up and into the computer is totally anti-climatic.
Yes, I did that in one turn. The "final room" was so different from the rest of the base that it was obviously the ending.