Gameplay Proposals/UFO Interceptions
This is a description of how I believe UFO interceptions should behave. I believe the current UFO interception scheme, which happens directly on the geoscape, is crude and insufficient. It can not effectively deal with multi-UFO flight groups, and the range at which combat happens is absurd, not to mention contrary to the UFOpaedia's weapon descriptions.
UFO interceptions should behave similarly to the ones in X-COM in the sense that an intercepting aircraft should attempt to rendezvous with the target UFO (or UFO flight group). If the interceptor succeeds in attaining the same geoscape coordinates as the target, the interception is successful, and combat commences.
Combat happens in a separate interface. This interface is a close-up of the immediate area, showing only a few square kilometers. In this close-up view, the UFO(s), interceptor(s) and weapons are shown flying around in actual combat. The player can actively influence the battle by giving his interceptor(s) orders in this interface.
Combat versus the geoscape
While combat occurs, time on the Geoscape does not stop. It is possible for new missions to appear or research to finish while the player is busy dealing with a combat scenario. However, since combat happens at a low time setting (5 seconds to 10 seconds, most likely), this is not very likely. The most notable effect of this concurrent time flow is that it is possible for new objects to enter combat, and craft to flee from it. For example, if the combat occurs within range of a ground-based weapon, such as a base defence facility, then the projectiles or beams from this weapon can appear in combat and attack the UFO.
The player can also choose to leave the combat view and return to the geoscape. Combat will still continue, however, and can be over almost instantly if the player speeds up time on the geoscape. A player may wish to leave a combat if there is another combat going on at the same time, or to tend to some other important task.
A new mockup of the planned interception screen is available here: 
Air combat is initiated when interceptors meet UFOs on the geoscape, when the two groups are right on top of each other. No weapons fire should be exchanged between aircraft on the actual geoscape. Only ground installations can fire/be fired upon on the geoscape.
The geoscape interface zooms in on the interception site and becomes the interception screen, giving us a full 3d overview of the combat zone with a representation of the terrain below, much closer-up and more detailed than the geoscape. (In early implementations or if terrain is too difficult to generate, the terrain can be replaced by simple fog.) Once the interception screen is reached, zooming should be placed under the control of the player by way of the mouse wheel or keyboard. It should only be possible to zoom out a few kilometres during air battles; the player must not be able to go back to the geoscape without clicking on the 'back to geoscape' button.
Zoomed out to the maximum, the combat participants would be only icons flying around over the terrain or fog, essentially there only to show their location and heading (as well as craft-specific info as in the UI mockup above). Zoomed in to more realistic distances, we should be able to see the actual craft models flying around, getting behind each other and firing their weapons. Possibly we can even offer different camera positions, it'd be cool to follow over the shoulder of your planes as they cut through the chaos.
The player's view of the action should be fully rotateable -- you can look at the fight from the top, from the side or even from underneath at any zoom level. The globe and/or terrain should not be drawn when the camera is underneath it. The camera should be able to focus on any specified participant in the combat. By default it should focus on a 'central point' calculated to be the average X/Y centre between all combat participants, looking directly down on the combat at medium zoom, and the player should be able to return the camera to this default position at will.
All craft participating in combat are listed in the 'participants' box, as seen in the UI mockup above. Left-clicking a friendly craft selects it and makes it available to receive orders. Left-clicking on an enemy craft assigns the enemy craft as target to the currently-selected craft. Right-clicking on a friendly craft pops up the 'orders' menu, allowing the player to determine the craft's combat posture and at what range the craft should engage the enemy. The orders should be treated as strong suggestions by the AI -- since these combats are mainly AI-driven, the pilot AI shouldn't always take the player's word as law when it's clearly not the right thing to do, but should treat it with the appropriate weight. Default orders are Defensive Posture (first try to survive, then try to shoot down enemies) and Weapons Range (maximum range at which all weapons can fire).
The currently-selected craft is displayed as a paper doll in the 'craft' box. Below the paper doll is the craft's weaponry and equipment, and below that is the craft's target. Any listed equipment can be right-clicked to turn it off or on.
The main UFO will attack any interceptors if armed, will try to disengage if unarmed, and generally wants to get back to its mission as defined by the campaign framework. If all human craft or the main UFO are disengaging, then any craft that stray too far from the main UFO will leave combat. These craft will have to catch up with the main UFO to re-enter combat. When not in combat, a craft can not attack. If all PHALANX interceptors leave the combat then the combat is over. This means that if the main UFO speeds up and the interceptors can't keep up, the UFO will outrun the interceptors and end combat.
A craft can fire a weapon at its target when the target is within range of that weapon and within the weapon's firing arc. Different weapons have different ranges and firing arcs. An interceptor will conform to the engagement range specified in its orders: Max Range (the maximum range of the longest-ranged weapon on the craft), Weapons Range (the maximum range of the shortest-ranged weapon on the craft), or Kill Range (the range at which distance modifiers to accuracy are smallest). Depending on the weapons the player enables or disables in the 'craft' box, this means the interceptor can fight at many different ranges.
Interceptor/UAV behaviour is also dependent on their player-selected 'Posture'. Available postures are: Aggressive (try to shoot down enemies, all other considerations are secondary), Defensive (first try to survive, then try to shoot down enemies), and Disengage (try to leave combat).
Any UAVs joining the combat with their assigned interceptors start out 'slaved' to their interceptor by default. When slaved, they attack the interceptor's target and protect the interceptor's rear as best as possible. They can be toggled out of slave mode by the player, 'unslaved', at which point they can be assigned specific orders by the player which they follow as best as possible. UAVs automatically become unslaved if their interceptor is destroyed. Individual UAVs can be slaved or unslaved by double-clicking on them in the craft list. Double-clicking on an interceptor will toggle slaved/unslaved status for all UAVs assigned to that interceptor.
Hitting and missing
In combat, being able to hit a target is a major factor. Quite a lot of attacks should miss their target. Since UFO interceptions are much simpler than tactical ground combat, it is not necessary to include a complicated accuracy model. It suffices to calculate a to-hit chance depending on a few factors (base weapon accuracy, countermeasures used by the enemy, distance between attacker and enemy). Then a simple roll is made, and depending on the outcome the animation on the combat screen shows a hit, a miss or an attack thwarted by countermeasures.