I've tried UFO:AI for a while now, and I have some thoughts, suggestions and reflections. These are not sorted in any way, and I realize that not all will agree with everything and that some of the ideas are probably underway anyway. Some comments will go beyond the originals, but there's nothing that says that you can't be better.
* I love it. It's like reliving the original games.
* It's beautiful, much neater than the original.
* Aircraft. As far as I can see, only three types are available, and new types can't be researched. A few more would be nice.
* I liked the "auto pause when something happened real time mode" of Apocalypse a lot, it removed a lot of the tedious ordering (such as moving a squad which happens to be at the wrong end of the map) and gave some nice "Shit! Shit! Shit!" moments when something unexpected happened. It also made gameplay faster, without losing detail. I realise that many players don't agree with me on this and that it would be a major rethink, but I just want to mention it.
* Interception. Some kind of visual depiction of the interception, instead of the geoscape, would add a lot. Apocalypse had it, since the scale was different. Perhaps some system like the old board game "Blue Max" or the newer board game "Wings of War", basically "Both sides give orders, then both sides move simultaneously, then whoever has an enemy within their fire arc shoots". Gives a nice dogfight feeling.
* 8 man squads? I miss my 36 man attack teams! OK, I can see why 36 might be difficult to balance, but a bit more than 8 would be nice and allow for more flexible tactics. Maybe 12 or 16? This could also be a way to balance tougher aliens if, as in the original games, coupled to the need to reseach larger aircraft.
* Nighttime geoscape. Simple, yet beautiful.
* I think I'm able to stuff any amount of aircraft into a single hangar (of the correct size).
* Grenades are way too wimpy. I miss the big explosions of the original games. With the current radius, I practically have to stuff the grenade down their throat to get them inside the radius. Grenades should be area clearing weapons. I want to be able to clear a room by having some grenade launcher guys launch a volley of grenades into it.
* I'd love to see multiplayer campaigns. Each player have their own bases, then when a mission begins, the soldiers are divided between the players so everybody has something to do.
* I'd love to see aliens vs human multiplayer, both in missions and in campaigns. A campaign would probably need to bend the story a little, for instance, the aliens have successfully taken over North and South America at the start and have bases there. For balance, humans would lack technology in the beginning, but have plenty of troops, while aliens are superior but few. As the campaign progress, they have the opportunity to overcome these limitations through research.
* Some missions are almost impossible to win without losses. I'm currently stuck on the Italian town mission with two bridges over a canal. There is always some alien getting a nasty long range potshot in. I'm going to try it with a team with better long range capabilities next.
* Names are repeating and sometimes stupid (I mean, having Britney Spears as a soldier?!?!). We need a longer list.
* More stuff. I want more equipment and stuff from the aliens, even if it's not very useful. It adds flavour. Perhaps energy shield, increasing protection from the front only? Not high priority, though.
* Possible future expansion: The underwater stuff, like in UFO2. Would be even cooler in a multiplayer campaign, where the different players would get different responsibilities, effectively creating four teams on two sides (land human, sea human, land alien, sea alien).
* Stealth. In Apocalypse, there was some invisibility device. Some kind of stealth device, perhaps more effective the less you move, would make some difficult maps more manageable, and also allow more different tactics.
* Nice UFOpedia pages. Kudos to whoever wrote them.
* Larger UFOs. I liked the big ships encountered in Apocalypse. Perhaps at the end of the game even large UFOs escorted by smaller, or a strike force of several smaller UFOs, so that the player have a real need to keep building his airforce.
* I want wimpy aliens as well. Imagine getting swarmed by a group of wimpy aliens as you step round a corner. Each one can be killed by a single hit, but there are a lot of them. Somewhat like a swarm of brainsuckers from Apocalypse.
* The dropship has a cannon. It would be nice if it could be operated by a soldier, in order to clear the immediate surroundings.
* The civilians needs some samples when they move, such as "Oh my god, we're gonna die!", "The horror! The horror!", "The aliens! They are everywhere!" and so on. Not that important, but adds feeling.
* It would be nice to be able to switch base even when in a sub-window on the base screen. I always have to go through all bases to find the one that just finished a research project, and having to go back is a bit irritating.
* A lot of odds and ends that needs to be finished, such as psychic abilities, implants, heavy equipment, gas grenades, proximity grenades and so on. I'm sure it will happen, so I don't worry about that.
* Much more variation in the maps than in the original game, with the possible exception of Apocalypse. Nice!
* Reports on the results of interception is a bit to discrete. It's easy to miss the loss of an aircraft.
As you might see from my comments, I personally liked Apocalypse best of the originals...
Overall, though, an excellent effort and easily one of the best open source games out there! Well done, keep it up!