Some core components must exist to qualify game as XCOM successor:

* strategic view (bases on planet, city, galaxy, star system)
* squad level combat
* research and technology tree

I suppose all games can achieve that but some little things create unique atmosphere, giving you reason to continue playing. Those are small things but with big effects. Let’s see some:

I am a big fan of the XCOM series. Released in 1994 „UFO: Enemy unknown” was never surpassed in playability. If I counted spent hours per game, „Enemy unknown“ and it’s sequel (or expansion) „Terror from the deep“ would be on top. 13 years later there is still no successor on sight, game which can take original idea, improve on it and use current technology to give us even better game.