I'm going to run a homebrew campaign centered around constructs, I've already come up with:

• big scary tank robot that's better run from then fought 
• two swarm robots that come from factories in droves
• extra-smart robot that knows spells but is physically incapable of walking
• concepts for a stealthy robot and slightly smaller big tank bot that can be fought

I probably don't need to many ideas but i feel like something is missing...

any ideas?

I'm stuck in the medieval times with D&D, so my idea is less "robot" and more "Iron human-shaped sentient being of mysterious Wizard origin."

The stupid thing believes it's a human and has developed the odd belief that it must restore body to flesh in order to be whole again. It will murder you and take your body parts, replacing his metal limbs with yours.

And since it's a Wizard's plaything the monstrosity can cast spells! What spells and what level he casts them at are up to you.

Flying Spy-bot that searches out threats then calls big robots to come deal with them.

Worker-bots that don't need to be fought as they only do their pre-programmed work like mining, building things, gathering materials.

Repair-bots - robots that fix, destroy for parts, and build new robots.

carrier-bots - big robots with swarms of little robots inside them

diplomat-bots - robots good at talking with people who deal with negotiations or manage humanoids that work for the robots.

trap-bots - robots that capture defective robots and take them to the repair bots.

Ebberon: Rising From The Last War has stats for a Warforged Collosus, that was basically a giant robot mech that people could go inside and pilot. It's one of the few monsters I've ever seen that comes with a battle map (showing it's interior space).

It would be great as a big boss battle. 

Robot that can cast spells should cast heat metal on nearby robots, causing them to deal additional fire damage on contact

I've completely forgot about support bots, good catch w/ the spy-bots and the repair bots, the other ideas can be given to the other bots I've made. 

Quote from CharlesThePlant >>

Ebberon: Rising From The Last War has stats for a Warforged Collosus, that was basically a giant robot mech that people could go inside and pilot. It's one of the few monsters I've ever seen that comes with a battle map (showing it's interior space).

It would be great as a big boss battle. 

 I've already got a gargantuan bot planed for a giant boss fight but thank you for bringing this to my attention as i could give one to the players temporarily to break through a massive army at the foot of a bbeg's lair depending on how it scales.

The archetypal several weak bots that combine into a big bot.

I mean ... I would argue that in DND, a 'robot' is called a 'golem'. But anyways: I've always liked cyborg assassins, faceless black machines with forearm blades, stealth and the ability to cling to ceilings and run on walls. I also like foldable combatants, neat packages with carrying handles, that unfold into killers composed entirely of gears and blades. (alternatively, they stack handily onto a self-driving cart from which they deploy). 

You could also do the T-1000 thing - a mimetic polyalloy, basically a sort of quicksilver Mimic.

here you go.

a basic robot that's been implanted with a false hydra's voicebox

a robot that that has an empty stomach cavity for storing captives

a robot that is like the t-1000 and can shape itself into any form, even down to color

the wyrm. a massive burrowing robot that basically works like a battleship. its got deactivated robots charging on its sides and when it surfaces the robots activate. its more of a mini-boss, something you have to deal with before the BBEG

rreeaallyy stretching the definition of robot here but you could have an intangible A.I that can possess electornics