I've often thought that after a crashed UFO there should be a random number of alien survivors, between 0 and the max number of aliens the ship can hold. I do like having the max aliens to kill (more stuff to sell), but it would make sense that some would die in the crash.
It might give more reason to shoot UFO's down for some people, especially harvesters. If you shoot it down, there will be less aliens and quicker fights, which some people like. Catch it on the ground, more aliens to fight, tougher/longer battle.