I have the following suggestions
1. Alternate Realities
First of all, I would like there to be more options. A long list would substantially increase playability, such as the list in Endless Empire or SMAC or the Civ IV mod Fall From Heaven 2 (the best game of its type ever made).
It would be especially nice if the game did something that SMAC could have done but did not: code alternate realities. Why not be able to go into a game not knowing whether or not psionics exist, whether or not the lowest enemies have 3 HP or 5 HP?
It would be nice if the aliens could have different goals. They might have the goals in the original or new XCOM. They might want to harvest something specific from the earth. One of my favorite unexpected moments in the original XCOM was walking into a kind of alien ship I'd never seen before and finding a cow on board (this time, please have it alive instead of on a dissection table).
2. Alien Vision
One key element of alien superiority was the fact that aliens could see farther than humans, especially at night. The original XCOM (released in 1994) made night fights especially dangerous, but gave humans light bombs in compensation. In addition, a flammable environment generated usable light, which was a significant innovation for the time.
One option would give the aliens better vision. A second option would give them better night vision. The light bombs could stun anyone using nightvision. Humans would be able to make nightvision goggles at the start of the game, but human nightvision would be inferior to alien nightvision.
3. Alien Bases
The original XCOM made locating alien bases one of the goals of the game. In the new XCOM, ships simply move back and forth within a nation's borders until you catch them or they leave. In the original XCOM, they had missions that would cross national borders, and you could eventually guess the location of alien bases, which were in truly interesting locations, such as Iceland or the Himalayas.
One suggestion is for an option changing alien movement. A second suggestion is to implement the base scouting mechanic from the original XCOM. The alien bases themselves were the most challenging maps in the game. There were far more aliens than on most maps. Running out of ammo was a real concern (I like the current implementation of ammo in the new XCOM). Bases were defended by powerful aliens and alien tanks. Bases had teleports, not elevators, so aliens could always appear behind you due to normal movement. Some aliens would have alien blaster rockets, the most deadly weapon in the XCOM arsenal.
4. Attacks on XCOM bases
In the original XCOM, human bases could be attacked. Human bases would be defended by every soldier stationed on that continent, allowing you to field more human units than in any other battle, and potentially giving all of your lower level units some combat experience. It would be nice to have these attacks re-implemented, but the current system uses only one human base.
It was especially nice that the layout of the base that you had built generated the map that you fought on. In the new XCOM, the base has many levels connected by a single elevator. I can imagine falling back, defending several levels. I can imagine aliens rescuing those held in containment. It would be excellent. I recommend an option in which the aliens may attack your base, understanding that the system may not be ready to have several dozen humans on the screen.
5. Levelling up
In the original XCOM, units levelled slowly but infinitely based on their actions. Experienced soldiers could even move further. They had better aim and reflexes. I recommend an option in which each successful action improves a statistic incrementally. I further recommend that soldiers have more statistics than just Will and Aim. Understand that this levelling system is subject to exploitation.
6. Retake lost nations
The original XCOM did not allow the player to retake nations that had surrendered. An option allowing this -- basically an attack on an alien base -- would be great.
7. Great design decisions already made
According to Gamespot, the XCOM designers have already re-introduced looting, added stealth, and written new mission types. They have re-introduced procedurally generated maps. I hope that the new system will allow humans to fire plasma and laser bolts at walls. This is not permitted in the current XCOM, and it reduces the human player's strategic options.
I hope that the mission types will go beyond the well-known tropes (escort, kill, find). The final two bonus missions in the Slingshot Content Bonus Pack DLC were delightful. I hope that the same game staff are working on the new game (failure to do this led to the terrible Neverwinter Nights 2 after the brilliant learning curve in original Neverwinter Nights).
8. If there's stealth, are there detection devices?
In the original XCOM, you had to devote two or more soldiers to using faulty detection devices which, like that scene in the movie Aliens, had predictable flaws, which only made the game better. If there are invisible aliens, there should also be detection devices.
Conclusion
I'm looking forward to the next XCOM. I hope that it's a step forward and not a step back (Just Cause 2 likely had no playtesters).
XCOM designers, please continue to focus on fun, but this time, allow the hardcore fans to choose options that will make the game more difficult and more complex.
