It's a fun concept. 
But what if X-Com travels in a helicopter? Or an Avenger? So not really practical, unless you have an idea how to solve it.
we can't check the craft upon arrival. And the mission is determined at the start of month, with location, faction and everything.
If you like the idea of a rare 2-part mission, here's one possible answer. Is it possible to create a map that has no flat areas without forest? The mission description is normal, but the briefing after landing states that the area of interest is inaccessible to ground vehicles and has no flat clear areas to land a helicopter. The player needs to reach the exit grid, clearing any possible obstacles. Of course, some armed obstacles quickly appear. Either that, or the briefing states that the base has posted a lookout, and agents need to reach the exit grid or neutralize the lookout in e.g. 10 turns. If they don't, the base would be abandoned and the lookout would flee. I imagine them something like this:
Zombie infestation description: "A group of aggressive 'Zombies' was spotted in this region. We must secure the area and make sure nothing escapes. Capturing any previously unknown living specimens is also encouraged."
Zombie infestation, mountains, part 1 briefing: "The area infested by zombies has terrain too uneven to drive a car or even land a helicopter. You have to proceed further on foot. Local police reports possible presence of hostile humans in the area, but the information is dubious. To abort the mission return X-Com operatives to the craft and click on the 'Abort Mission' icon."
Zombie infestation, forest or jungle, part 1 briefing: "The area infested by zombies has vegetation too dense to drive a car or even land a helicopter. You have to proceed further on foot. Local police reports possible presence of hostile humans in the area, but the information is dubious. To abort the mission return X-Com operatives to the craft and click on the 'Abort Mission' icon."
Zombie infestation part 2 briefing: "A group of aggressive 'Zombies' was spotted in this region. We must secure the area and make sure nothing escapes. Capturing any previously unknown living specimens is also encouraged. To abort the mission return X-Com operatives to the exit area and click on the 'Abort Mission' icon."
Monster hunt description: "People went missing in the vicinity, some were found butchered - seemingly by some wild animal. With help from the local forces, we have located a possible reason: some unidentified creature. We should identify this threat and put an end to it."
Monster hunt, mountains, gold diggers, part 1 briefing: "The likely area to find the animal of interest has terrain too uneven to drive a car or even land a helicopter. You have to proceed further on foot. Be careful — there may be illegal gold diggers in the area, who may mistake you for competition. To abort the mission return X-Com operatives to the craft and click on the 'Abort Mission' icon."
Monster hunt, forest or jungle, poachers, part 1 briefing: "The likely area to find the animal of interest has vegetation too dense to drive a car or even land a helicopter. You have to proceed further on foot. Be careful — there may be poachers in the area, who will not welcome outsiders. To abort the mission return X-Com operatives to the craft and click on the 'Abort Mission' icon."
Monster hunt part 2 briefing: "People went missing in the vicinity, some were found butchered - seemingly by some wild animal. With help from the local forces, we have located a possible reason: some unidentified creature. We should identify this threat and put an end to it."
The possible enemies for part 1 may include:
- illegal gold diggers, who may mistake you for competition;
- illegal gem prospectors, who may mistake you for police;
- poachers, who may mistake you for rangers;
- smugglers, who may mistake you for borderguards;
- escaped convicts, who may want your weapons;
- armed deserters, who may want your transport;
- or various combinations of the above.
Cult base description: "We have located a major base of operations of {CULT NAME}. This building is heavily defended by numerous and well-equipped troops. There will be a high-ranking officer present, so take this opportunity to capture and interrogate them."
Cult base, part 1: The {CULT NAME} outpost is strategically placed in an area inaccessible by car and without spots suitable for a helicopter. You need to proceed on foot.
There is a forward observation point that should give an evacuation signal if they fail to stop you. You only have 10 turns to neutralize them, sneak or force you way to the exit grid to the main base. If you take too long, the surviving lookout and the base inhabitants will escape.
Cult base, part 2: "Your agents have reached a major base of operations of {CULT NAME}. This building is heavily defended by numerous and well-equipped troops. There will be a high-ranking officer present, so take this opportunity to capture and interrogate them."
