Bioshock creator Ken Levine was crucial to the development of XCOM 2

gamesradar.com:

 Bioshock creator Ken Levine was crucial to the development of XCOM 2.

Speaking to Edge Magazine, XCOM 2 director Jake Solomon said that a key idea for his sequel came directly from Levine: "The kernel of the idea - that the aliens won - came from Ken Levine. I stole it from him. We're good friends, and Ken is a very big XCOM fan. He was like 'take it, use it.' Having a strong theme like that led to stuff like very basic stealth and hacking. It was a huge project and took a really long time, a lot like Enemy Unknown. It had all these changes that weren't going well, and then of course came together."

When faced with creating a sequel, Solomon found himself struggling with the narrative direction of his sequel: "We were very happy with Enemy Unknown, but I had only ever designed that, and had no idea how much XCOM 2 should change. I was like 'the whole strategy layer is going to be very different. We're going to have base building. I want random, procedural maps. We had all these big changes, and because of that, XCOM 2 was difficult'."

In a previous conversation with Rock Paper Shotgun, Solomon covered the differences between XCOM and XCOM 2, and why creating a sequel was so hard. He claims that the root of the issue is to please fans, who he says "have more ownership over the title than I think the developers do. Then it becomes a matter of 'OK, we're sort of curating this franchise owned by the audience'." 

Elsewhere in his interview, Solomon outlined how his upcoming game Marvel's Midnight Suns is the complete opposite of XCOM 2.