Alright, here I go. Good ol' Metal Gear Solid!
More Hardcore: Enemies have realistic cones of vision.
This one is actually true on European Extreme in MGS2 and MGS3.
Anyway, Hardcore X-Com Mk. II:
-Chryssalids are now the exponential threat they logically should be. Fail a mission without killing all Chryssalids present and they will begin to massively grow in number until the nation(s) they were dropped within must be sterilized in nuclear fire lest Earth be overrun.
-Every nation on Earth is now represented within the Council, and conflicts between them are possible that will effect soldiers of relevant nationalities.
-When a nation becomes allium-sympathizing scum they begin to actively work against the rest of humanity, attacking both them and X-Com. Keep the nuclear club out of allium hands if you know what's good for you. Time passing slowly raises the control the aliens have over all nations.
-Elerium-115 is hazardously radioactive outside of a containment field.
-As the war drags on Earth's economy begins to crash into the ground, severely limiting what funding you might still have.
-Be careful what you sell on the Black Market. Who knows where those laser cannons might end up...
-The Skyranger can be intercepted and shot down by hostile craft, be they alien or human in nature.
-Injury times are now very realistic, with critical wounds taking months to recover from without....liberating certain advancements in medical technology from the alliums.
-Ethereals can exert mind control that only ends if they are killed. Killing a unit that is mind controlling one of your own causes psychic backlash which can be fatal. You automatically fail a mission if all living soldiers are under mind control.
-If you fail a mission in such a way that the alliums can recover your solider's bodies, they will gain any technological advances X-Com has made. If they obtain a mind controlled solider they immediately know the location of your base and will send the mindjacked solider(s) to attack it alongside their usual forces.
-Once the alliums confirm the location of your base their attacks will be frequent and increasingly brutal. Scientists and Engineers are at risk of death during these attacks and are defenseless aside from the chance that they might acquire a weapon.
-A solider panicking invokes a panic check on all soliders who can see them. Yes, this does mean a panic spiral is more likely than not to occur.