Just thinking could be very funny and open a lot of possibilities if the advanced grangers could dig / drill holes / passages on places of the map. I know it sounds crazy, but the amount of things that could be done.
To limit this maybe you could create 3D areas on the map where it was possible (areas of sand, wood, etc) and areas where no (metal, stone). Perhaps could be used as a 3rd evolution of grangers only accessible in lategame where aliens could acquire a other strategy against humans (a backdoor in their base)

I can imagine how aliens could create deep hives with numerous passageways dug in soil or wood (as I say, this could be controlled by assigning a limited area in 3D on the map where you could dig). This would give a new level to that of "building a base" (imagine a map where aliens start in front of a wood wall, and they need start do dig in the start of the game for have fast a safe base)

I suppose this is complicate of implement, but anyway I expose it. If just little areas were available for dig it could also we awesome.

PD: Apologizes for my English, it is not my native language.

haha that's very interesting idea but I don't think it's easy (not impossible) to implement this in UE3

Diging holes in common way (player will start creating tunnels) is currently not possible. Because it will need to modify not only three basic construction elements (bsp, objects, terrain) but also their collisions and that is problem.

Of course there can be done some tricks - you will create just some entrance, enter it and somewhere you can go out. It will have still many problems - where you will go out on complex maps? Also humans will not be happy when you will come through their defence

Interesing solution is in game Plants vs Zombies 2 - with one class you can behave like a mole But still I think that can be too strong "weapon" for aliens.

In natural selection 2 , beast's builder can build 2 structures that act like a tunnel. When you enter one, you are getting into biotic tunnel that you have to travel thu to jump out from the other entrance.
Its very nice tactic complication . 

imp: I saw it on video, but it will be probably trick. You will enter one structure = you are teleported into tunnel that is completely outside of map and at the end of tunnel you are again teleported in place of second structure.

Anyway, there are mostly one floor maps = more posibilities for such things.

Rota: Yes of course i realize that. But who cares. its still shortcut to another location and srsly i think you guys should consider something like that because it can make game whole lot more interesting since it would add tactical complexity and i think its fairly important.

imp: yes it can be good thing. We can consider it later (after final release), it looks like not much work.