I'd rather see specialized alien attack crews: mixed alien crews which always contain a few silacoids... why silacoids? because they can burrow, of course! I mean, think about it, they are rock creatures with a super-hot core. of course they can eat their way through rock. so while 70-80 percent of the aliens would go through the hangars and access lift, smaller squads would follow the silacoid tunnel-maker and attack any xcom facility next to solid earth by breaking through from the outside.
that would mean a complete change of base setup for defendability, and a real tactical challenge. imagine your troops all rushing towards the hangar/access lift which you so neatly arranged to create a choke point, waiting for the first alien to poke its ugly head around a corner, when suddenly five or six chryssalids attack them from behind. oh, the terror!