Ive played the original Xcom ones and the altair ones and the xcom remakes and all have elements i liked and all had elements i didnt like.
The early Xcom had the weird abduction events and it felt a lot like you were stumbling in the dark trying to work out what was going off, somehow those feelings haven't come through much in all subsequent games. I didnt like the last xcom expansion as it tried to force you to rush missions which generally is a bad idea.
I loved the destructible scenery in the apocalypse but i hated the fact you were stuck to one city. I liked the corn fields you could burn to flush out enemy in the first one, and i liked fighting in the tankers in the second one.
What i'd like to see is a true multiplayer campaign where the aliens player has to achieve x amount of intelligence gathering,resource gathering etc to build more ships to complete the invasion.
Think of a sleeper ship with crew on ice, sent over distance and have enough gear to set up the basics of colony, maybe it would be fine but in the years they have been traveling the planet has evolved complex life and the resources arnt sufficient to fight a war. They have technology but struggle for resources the humans have the basic resources but technology is way below.
Aliens will try everything to take the planet, They have many types of missions. ( Food gathering.. Associated missions .. setting up fattening stations to bulk up the foodstock.. Infiltration by Aliens dressed up as colonel sanders to get the process started.. lol ), Taking over food production or additives to pacify population. Taking over politics, taking over media, many options to infiltrate and the human player gets lots and lots of events/news and they choose what to investigate. Some abductions may turn out to be kids off playing or human murder just to throw things. Have you seen dark skies? That's the period, post war and the political missions.
Asteroid mining and moon bases would be required to refine 'specialist resources' so aliens would start with limited weapons to start with and be forced to work hard to develop their war machine.
That would make the overall world campaign vary and add elements of human strategy on both sides.
