I am not sure how you are going to make the game AI to advance to harder ufos/aliens as the game goes on, but please, please try to make it smooth like it was in the original x-com.
Some years ago I remeber playing a game an x-com remake (I forget the name) and there harder aliens were spawned after a certain research point. As I remember tier 2 or 3 aliens got on board as soon as laser rifle was researched and the game became insanely hard if you did not know it would happen and be prepeared. This also led to abuse of the engine since player could leave a certain action/research aside until (s)he felt ready for the next "tier" of aliens.
It would be more realistic if harder/top level aliens would occasionally show up in the early stages of the game and even possibly force the player to evacuate all troops so not to get totally over-run.
One memory from x-com was a night fight in the jungle againts a shot down terror ship full of ethereals and 2 sectopods. I had barely laser rifles and personal armour for my soldiers and those aliens gave me a run for my money...One of the rare occasions that I did actually evacuate and let that site disappear from the map. (I was never one to abuse save/load if things went south)
Also weak aliens should sometimes show up at the later part of the game when you can sometimes just wipe the floor with them with all the high end weapons.
I hope that made any sence...I am looking forward on this project!
