Alternatively, it could essentially be a 'landed UFO' mission, focusing on the UFO that is coming to assault the Terrans, but the player's dropship tile uses a crashed version.
sounds fairly simple to do from a mappers perspective.
certainly the Aliens would take interest in the only "resistance" around. they might try and snatch downed personnel for study. But I propose this would be a mid-late game behavior. Takes awhile for the aliens to notice PHALANX and take interest. Perhaps this would make a good precursor mission to a PHALANX base assault?
(seems the aliens learned something from the troopers they captured.... Don't ask how... it's messy.)
as for how the mission should go:
From a military perspective. in the event of a downed vehicle your not sending in SAR (Search and Rescue) right away. while a priority, SECURING THE CRASH SITE is the first priority. Send in a vehicle to SECURE the site fully loaded with troopers and gear. they set-up a perimeter, secure the site, kill any aliens around and begin initial Medical care and recovery of sensitive items and take control of the scene. Then the SAR types move in with specialized equipment to effect recovery of equipment and medl-evac of wounded personnel. I assume that local Militaries still exist? They have Equipment and SAR/special forces troops that can be pressed into simple recovery work. Especially after PHALANX has secured the site and collected the sensitive information. No real need to simulate that part of the operation. Just have a friendly craft land and "secure" the site for several hours. If you simulate actual flights for "transfer" of supplies, just initiate a "transfer" of recovered personnel and equipment after the "secure crash site" mission ends.
what this means in game terms:
Scenario 1: is that the aliens may assault the crash site before the cavalry arrives. in which case you defend the crash scene with what you had on the crashed bird. (minor mechanics with wounded troopers before the fight/ lost equipment aside.) Put a time limit on it to represent the arrival of the rescue party and the aliens bugging out because they don't want to be caught with a UFO on the ground with PHALANX craft overhead shooting them up.
Scenario 2: Cavalry is on scene holding the Crash site, since they made it first. Aliens assault the crash anyhow. typical UFO on ground mission but you have TONS of troopers to fight them off. you might only map a few maps for this or random generate them if you can. since your talking about a whole PHALANX DROPSHIP, Wreckage, and a LARGE/Assault UFO. This would really only be a late to ENDGAME type scenario because it means the Aliens are picking a FIGHT with PHALANX instead of just trying to do a snatch and run.
scenario 3: Phanlanx Dropship arrives and secures the site, sits for several hours to recover craft/Personnel. Mission ends, no aliens show, routine transfer of recovered items to home base they came from generated and happens in the background like any normal transfer between bases.
pretty straight forward.