@OP: I like the idea-little/weak robots should be dretchable
-and should kill naked humans easily
-also, maybe some different weapons than the Tremulous Human weapons?
-bring back Quake super-shotgun/nailgun/railgun/nadelauncher?
-(
OR Unreal blaster/crystalgun/shockrifle/flakcannon?)
You
could have
four stages of Robots (
both playerteams at S1, S2, S3, or SD):
Shotgunbot -
guard Factory, hunt players: has four-barrelled gatling shotgun, slow reload, low hit points (50?), very dumb, slow movement, cannot wallwalk, small explosion when killed (like 1/2 as much as a turret exploding), turns if it bumps into something, susceptible to ground headbite & regular headshot, three-minute charge, 30 second recharge, 15 second re-equip, defenseless when out of ammo, player gets healthboost (25%) for destroying more than two within own build line (power/creep) in one spawning/one minute (Stage 1) new shotgunbot every three minutes (robots initially equal to total number of players on both teams; e.g., a 5v5 would have ten bots with a new one every three minutes), size of crouching Human/Granger
Nailgunbot -
hunt players, escort Repairbots: has nailgun, medium reload, medium low hit points (75?), smarter, quicker, cannot wallwalk, medium explosion when destroyed (like a turret exploding), turns if it bumps into something, susceptible to helmeted headshot, dretch anklebite does 70 damage to nailgunbot, 5 minute charge, 45 second recharge, 30 second re-equip, chainsaws/sawblades when out of ammo, player gets speedburst for destroying more than four within own build line in one spawning/two minutes (Stage 2) new nailgunbot every two minutes, size of crouching Human/Basilisk
Railgunbot -
hunt players, escort Powerbots: has railgun, long reload, medium high hit points (100?), not too smart, can wallwalk?, wallwalks if it bumps into something, toggles wallwalk off randomly, mild radiation when destroyed, looks/acts like Deus Ex spiderbot, likes to snipe?, can be dretched/basi'd (just like a turret in that respect), also has blindspot above & behind, 7 minute charge, 1 minute recharge, 45 second re-equip, can overheat and explode causing massive radiation damage, radioactive when out of ammo, player gets teleported back to own base for destroying more than six in one spawning/three minutes (Stage 3) new railgunbot every minute, size of crouching Human/Marauder
Nadebot -
hunt players, follow wounded enemies: has nadelauncher, very long reload, high hit points (150?), not too smart, chases players, cannot wallwalk, large explosion (like a nade)
and radiation if destroyed, battlesuit level of head armor (except for one tiny dretchable weak spot), 9 minute charge, 3 minute recharge, 1 minute re-equip, hot as turret slag when out of ammo, player gets teleported (& gets 10 second stillshield) to random location for destroying more than eight in one spawning/four minutes (Sudden Death) new nadebot every thirty seconds (you should have destroyed the Robot Factory a
long time ago), size of battlesuited Human/Dragoon (can't fit in every vent, only some)
All bots have limited ammo (say, the default pickup Q3a value for that particular weapon) and lifespan (as above, three to nine minutes), after which they
must return to their Factory to recharge and re-equip. They will begin heading back one minute before they are out of charge, and will stop in their tracks when they are out of juice. The recharge would take some time, and the re-equip would be loading up ammo. They would have to return to the Robot Factory to recharge their powercells whether they had used up all their ammo or not, and will visit the Ammo Station whether they have expended any ammo or not.
At Stage Two, the Robot Factory will send out
Repairbots, (Shotgunbot-sized Robots without weapons) to repair the unmoving but undestroyed bots and give them a field charge which is only good enough to get them back to the Factory, where they must recharge and re-equip before returning to the field.
At Stage Three the Robot Factory will send out
Powerbots, (Nailgunbot-sized Robots without weapons) which will re-enable all unpowered bots within their range, and home in on them, to provide a field recharge and a field re-equip for Railgunbots.
At Sudden Death the Robot Factory retools any remaining Powerbots into
Ammobots which can provide a field re-equip for Nadebots, and which have a huge explosive animation if destroyed (like three nades in succession, bl-bla-blammm!!!).
Both Repairbots and Powerbots would be relatively unprotected, except that Powerbots would have an electric shock like the Reactor/Adv Mara and be escorted by Railgunbots, and Repairbots, while essentially defenseless, would be escorted by Nailgunbots.
After Stage 1 the Robot Factory will defend
itself, and spawn new Robots whose
first directive is to
hunt players (or fix other Robots). Ammobots are entirely defenseless, and will thus be blowing up all over the place.

At the stage changeover for Robot Factory retooling, all bots would have to return to the facility by the end of the retooling period, or they would cease functioning until reactivated by a Repairbot or Powerbot. (Too complicated?).
Of course the server would slow down if it had to render all of these actions (you might have to reduce playerslots to make up for the visual lag of all these AI bots and their weapon effects). During their trip back to their Factory (and I like whitebear's idea of making each successively better Robot have a new and discrete origin, but wouldn't maps get crowded?; maybe they should just upgrade the Robot Factory like the OP said), the bots would either be defenseless, or maybe have chainsaws or be hot or radioactive from expending so much ammo so rapidly.
Almost last point: although the Robot team's kills don't count for the Humans or Aliens, they do count for the Robot Team, right (is that how they stage up, or would it be time-based, or possibly based on when both playerteams stage up)? What if a Human or Alien kills a Robot? I know it doesn't count toward stage-up, but how is it recorded? And can the Robots annihilate a playerteam? Can the Robots knock one or both client-controlled teams out of the game and win?
One more thing: Robots go
into Factory for recharge/re-equip?
OR remain outside and somewhat vulnerable? Small Aliens can enter S1 Robot Factory or crouching Humans (original Shotgunbots are about half as tall as a standing Human). Factory has defenses one stage weaker than robots it produces (initially defenseless, then has Super-Shotgun Turrets, then Nailgun Turrets, then Railgun Turrets). Factory shuts down for retooling at stage-change (first 30 seconds, then 45, then 1 minute). Factory begins producing Robots as players join teams (default set at one Shotgunbot per player). Robots which survive any Stage must return to factory at stage change for retool. New Robots produced at an accelerating rate (one every 3 minutes in Stage 1, down to one every 30 seconds in SD).
Also, both playerteams want to destroy the Robots and their Factory as early as possible, when they are weak, thier factory is defenseless, and the enemy gets little benefit from destroying them.
Lastly, how big would it be, and in all of the default maps, tell me where the original Robot Factory default location would be. Like as big as an Armoury to start, and as big as a Reactor by the end? And on, say ATCS, where would it go at mapstart? In/on the central building? Where in the other default (and popular) maps?
List, if you please, sir.

Oh,
another thing. Robots stage up automatically when weaker team stages up (so if Humans reach S2 first, Robots would stage up when Aliens achieve S2), with this exception: Robots can advance to the next stage for getting twice as many kills as it would take a playerteam to achieve the next stage. If it would take the Humans or Aliens twenty kills to get from s1 to s2, the Robots would have to get forty kills to go from Shotgunbots to Nailgunbots. Any forty player kills, regardless of which team feeds them more.
Fun idea.
Thanks for letting me warp it out of all proportion to what you originally intended. Cheers!
Oh,
and... the Robots are the creation of a rival interglobal corporate cadre/cabal which put all of its money into robotics for space-mining instead of clone-marines doing blackops wetwork for private interests in the nearspatial realm. They're using the war between the Humans and the Aliens to establish a cyber-hegemony of Robots, with Nadebots as their ultimate expression of technology.
Or you
could have them using Unreal-type mining tools instead of Doom weapons (
sacrilege!), like the U1 silly blaster or that crazy "machine gun that shoots crystal shards" or the shockrifle or flakcannon:
Blasterbot (a security guard/nightwatchman kind of Krazy Kopbot);
Minerbot, with the U1 "machinegun";
Shockbot, a wallwalker with a lightning gun (another security device); and
Flakbot, the ultimate guarddog.
Possible mod names: QuadshotBotkiller, RoboHunter, Cyb3r514y3rz, Abandoned Space Mines and the Robotic Killing Machines Who Dwell There: The Dark Cadre Initiative or maybe just
Robot Factory (my favorite)
kthx again
enjoy