We have already implemented stamina, and the heavy breathing effects- so no worry about that. The heartbeat, would work great for some added atmosphere and will most likely be implemented- kickass idea.
The digging thing? Can't we just have a droid go semi-invisible or something? It's not like I'm not considering the idea, but it does sound far fetched. I'll write this idea down for when tremulous is released, extra abilities we can have for another class branch (besides offensive, defensive, and builder). Of course, no promises... but the idea is surely interesting and would effect the overall gameplay slightly.
The independent swarm deal- curious as to what you had in mind to this. Small droids would be hard to hit, and the damage they make would most likely be miniscule. Do these run or fly? Are they fast or slow? Are they lethal or just a mere annoyance? Anyone's welcome to fill the gaps.
The "necrotoxin" idea you mentioned would work with a type of "grenade" that created a dense cloud of toxic gas in it's wake. This ability would have a more beneficial effect offensively, and would also cover their tracks when the need to escape quickly occurs. We have tinkered with the option of "manipulating" the type of gas released, whether it affects stamina, vision, speed, health- the options are limitless.
On a small note, Necrotoxin doesn't do any damage to the individual affected. The Necrotoxin (*mental note: name is too sinister) simply lays inert in the human body, until another chemical administered during infestation creates a chemical reaction, causing the human's flesh to melt into a malleable form and be added to the droid's own body. Huyah.
All in all, good suggestions. Thanks for the input.