I know it's too early to talk about it, but theoretically, Firaxis probably is already working on it, because it really has to launch something on 2020 after they end up with Civilization 6's expansions.

Many fans say that XCOM 3 will be a reboot to Terror from the Deep with underwater battles against primitive aliens that have been idling 65 million years through the depths. Now I do not know if current fans will be able to taste battles fought between diving soldiers and aquatic primitive aliens. But I think something nice will come out.

XCOM 4 will be a reboot to Apocalypse, with the action set in an apocalyptic future when the terrestrial ecosystem has been severely damaged, Terra becoming a true Ecumenopolis and humanity has begun to colonize Mars.

What would I expect from the next XCOM regardless of "setting":
  • the return of mech troops and genetically engineered soldiers (even transformed into mutants just like Civilization Beyond Earth)
  • A global map where we can take strategic shots like in Civilization.
  • build and maintain multiple bases located all over the globe, even to manage terrestrial Cities, underwater colonies and space colonies on Moon and Mars
  • Rival Human Factions (regional governmental organizations or Multiple XCOMs) to fight against aliens, each with its fighting style, its specific culture, its units. At the beginning of the game, each player can select and chose any human faction to play. At late game, can be there the risk of civil war outbreaks between these factions in case you went on the bulb. Some factions may even ally with aliens without losing the game and leading the war against the others until it conquers everything, gains supremacy and becomes a global organization that installs the dictatorship, living aloft with aliens, integrating them into society. Yes, you can ally with the space invaders without losing and winning the game by waging war with enemy factions. If it is possible to even implement a co-op mode in your campaign to play against another player who has chosen another faction that will either be allied with you in a common war on aliens, or you will be an enemy, and you will lead a war on all fronts, both to erode your factions while aliens look at you, then surprise you both with a surprise attack and destroy you on the principle of "Divide et impera ". Or even ally or ally with aliens against each other. Eventually, there are also rebel factions which fight with both government organizations and aliens.
  • Alien Mercenaries - Temporarily recruit aliens outlawed by their peers and use them in battle to counter enemies
  • VEHICLES! Yes, I would like to see tanks implemented in this game, why not? Aliens to use their armored vehicles, people to use rockets and rocket launchers, maybe even soldiers riding on mopeds and even improve them along the way
  • AIRCRAFT (the humans with bombers and supersonic jet, aliens with their space ships) - as options to call a target (as in Company of Heroes), but at a lap time, as aliens will have the right to bombard you in turn with their mother ship, so you need to know where to shelter your soldiers in advance just as they will have the opportunity to shelter their troops when you bombard them
  • Naval/Underwater Battles: why not? I really want to see how it looks like this, and I'm really curious if Firaxis would have enough imagination for that.
  • LONG WAR, but in Hardcore mode, for centuries or even millennia , bloody wars for resources and supremacy between human factions, rebels and aliens on Earth, Moon and Mars!
  • Weapons of mass destruction - if you are well with the money and you have managed to build one and see that in a certain battle, the situation may be ...stinky, you may have the option to evacuate your soldiers and ask your base to launch a NUKE or a biological bomb on the city infested by aliens. Being irradiated or infested by viruses, neither humans nor space invaders will be able to master that city. Both lose the match.
  • Space battles - this may be in XCOM 5 that will be inspired by Interceptor, but in TBS format like Sins of the Solar Empire, Alpha Centauri, CIV Beyond Earth or Galactic Civilizations.
    Multiple victories like in Civilization Beyond Earth : Alien Domination, Transcendence, Supreme Victory and annihilation of invaders, Technological singularity to counter invaders or Diplomacy - make peace with aliens and permit them to be integrated and be part of human society or make peace with other factions to counter the aliens