Something gameplay wise is not boring/repetitive when the stakes are high, the players needs to constantly adapt (UFO fight encounter involves part random events, part player skill, part pilot skill), the game art is beautiful and when there are enough complex systems forming a whole and those systems are synergetic and have important implications during that particular event.
Exotic materials scarcity
The materials needed to greatly advance your tech via R&D should be acquired only from rare artifacts found only in UFOs. Not all UFOs are destined to have them, whatever their class.
UFO interception scarcity
Intercepting and being able to intercept an UFO should be a big deal. The majority of time, (more so at the start of the game) the player will not be able to catch up if them.
Tension derives from uncertainty. An UFO must be relentlessly pursued by scrambled planes from different bases that are strategically placed.
Planes should be able to tail an UFO until others from other bases can join the fight to increase the odds of success.
This will put the player in front of a dilemma: wait for others but risk to lose the UFO or risk to be shot down (or sacrifice precious materials for repairs) but get the prize.
No interaction with UFOs will come lightly. The possibility in gain of exotic materials will always cost large amounts of conventional resources and sometime pilot lives.
Boredom always arises when there’s player complacency and lack of tension.
Every UFO class/subclass has its special traits/maneuvers but not always.
Fallout 1's combat became boring relatively fast but what never got old were those death animations accompanied by exquisite sound design.
Art could be stylized but planes and UFOs are in 3D. Hits must feel visceral, player receives visual & audio feedback, hits leave marks, planes physically react to hits, background reflects the area on the globe where combat takes place, etc
Random events, pilot & external factors
Unexpected engine failure
Pilot blackout while maneuvering
Overly aggressive UFO
Pilot adrenaline rush
Pilot panic/ untimely ejection
Weather (storms/lightning strikes, poor vision, increased failure of planes, increased time for lock on)
Time (day/night combat implications)
Altitude (high altitude = poor performance for specific engines)
(low altitude,=possibility for friendly anti-aircraft battery intervention,
higher plane frame stress during maneuvers)
Pilot stats (influences random events)
Endurance (constant performance output)
Spatial awareness (impressive maneuvers, extreme ejection, push the envelope)
Mental toughness (panic resistance, feats of bravery)
Zealotry (kamikaze tactics, self-sacrifice)
Combat takes place in three dimensional space.
Combat involves simultaneous turns ( gameplay ethos similar to Phantom Brigade )
Combat involves different stages (long range and close range/dogfight)
Long range: firing long range missiles and maneuvering for optimal starting areas for next stage.
Close range: using short range missiles and autocanons.
Players has access to predefined maneuvers.
Player input is by combining different maneuvers depending on the situation.
Newer planes and high pilot skill unlock more evasive complex maneuvers.
Planes engagement duration is limited by the amount of fuel when engaging in combat (longer pursuit, lower on fuel, more agility)
Player can trade fuel (external tanks) for agility.
Player has the possibility to keep planes on mission past the point of no return (during UFO pursuit or actual engagement) by sacrificing the plane but not the pilot (ejection).
Ejection always has a chance to injure the pilot and even a lower chance for death (ejection system failure)
Player can trade chance to hit for security.
Longer aiming time, more chance to be hit by the UFO.
Player turn has a countdown.
The firing arcs/zones of the UFO weapons are more broad as the weapon damage is lower.