First off, my sincerest apologies if someone has already posted something to this regard but I was just struck earlier today about how many of the alien structures exhibit characteristics that could be adapted into a mod styled on the beloved Plants Vs. Zombies by Popcap.
Admittedly I thought of this idea first in jest but as I think on it some more it seems like a perfectly fun little game could actually be made using the idea. All of the basic unit types from plants vs. zombies have some analogy in the existing NS2 Alien structures. You have Harvesters that could fulfill the role of sun flowers. Whips would serve as a wonderful blend of both melee and ranged offense/defense similar to having a Venus fly trap, umbrella leaf, and a melonpult all in one. Hydras could act as a basic low level offensive counterpart to Peashooters. Cysts with their mature state rupture ability could act as a potato mine. Clogs could replace Walnuts. Then of course there are the other structures that could make new but certainly plants vs. zombies inspired gameplay that fits with their existing roles in NS2. The Crag could naturally heal and reduce damage taken by the alien "plants" in addition to spawning waves of babblers to attack the enemy. The Shade could disorient and thus slow the progress of marine "zombies" as well as cloak structures and create phantasm structures to distract and stall the enemy. The currently unfinished Shift structure will likely possess many other abilities that could be adapted for the mod without much effort. Perhaps even the alien tech structures might also find some use as determined by the community. Even the alien classes could potentially have a place in the game. I for one think a charging Onos or crop-dusting Lerk makes for a perfect replacement for the lawnmowers, pool-cleaners, and roof sweepers that clear the lanes that the "zombies" manage to break through. As I hope is clear to see, the alien team holds many units ready to serve the purposes of making a Plants vs. Zombies mod.
Of course the marines themselves hold great potential for a mindless zombie horde. From the noob "zombies" running into a fight wielding only a switch axe all the way up to the jet-pack marines flying over enemy structures and the mighty exosuits devastating all that lay in their path, the full range of marine units and their accompanying MAC and ARC robo-buddies would make for a "zombie" horde almost as diverse and deep as the ones featured in Plants vs. Zombies itself.
As it stands, I am no modder. I also know that the odds of some member of this community picking this project up and making it are slim to none. That said, I am still so very thankful that the UWE developers have given this community the tools to make mod ideas like these not only a possibility but a reality for anyone with dedication to make it happen. I hope that my idea has managed to bring a chuckle to at least some of you and if by some remote possibility any of you are inspired to pursue this as a personal project then by all means do so. You not only have my explicit permission but my encouragement. Either way, you all keep doing the wonderfully creative things you do that make modding one of the best aspects about PC gaming.