With a little tinkering it is possible to turn ‘Robot of Sherwood’ into a new adventure involving the Cybermen. A great advantage is that the story already includes robots from space and the Sheriff reveals at the end of the story that he is a cyborg.
A spaceship crashing during an historical period is the only essential element, allowing you to adjust the location and time period. The televised story does make a good case for placing robots in a medieval period, where they can pass as armoured knights.
The Cybermen could be native to the time period. In this case they could be scouts preparing the planet in advance of Mondas. They could be the earlier models, more capable of interacting with others and not so driven to convert everyone (just those who it is logical to do so).
They could also have time travelled, either deliberately or accidentally (after all they did crash). This allows you to have more advanced models of Cybermen. This would also allow them to have databanks full of data about the future of Earth.
A big difference is that the robots main goal in this story is to repair their ship so that they can continue to head towards the Promised Land. Cybermen typically attempt to convert others but it could be that the Cybermen realise that Earth isn’t sufficiently advanced to suit their needs. In this case they could be trying to keep a low profile until they can leave.
If they needed to expand their numbers or replace lost soldiers they could convert guards loyal to the ruler they’ve allied with or convert captured villagers. These new recruits could stand out from the other Cybermen, as they have been gathered from poor stock and made with primitive tools.
There is a plot point about the robots gathering gold. In the original story this gold is used to repair the ship. Given their vulnerability to the metal the Cybermen could be collecting it to prevent it being used against them.
This could lead to a scene in which the Doctor (or your PCs) are placed amongst the other prisoners, surrounded by gold. The heroes can then show the peasants how to use the gold as weapons to fight for their freedom.
Gold arrows might therefore be a powerful weapon against the Cybermen. If you want to heighten their vulnerability the PCs could use gold swords to cut down their enemies. Greedy NPCs might not want the PCs to use the treasure in this way.
For much of the story it is the Sheriff that is the main villain. Keeping the Cybermen in the shadows (or disguised as the minions of the villain) can surprise the PCs. Their strategy will be to deal with the villain so when they learn his true nature and what they are really up against can require them to quickly come up with a new plan.
The Cybermen could find partly converting a local NPC a good tactic. It gives them an agent who can interact with the locals (and in some cases command them) while both giving them control over their agent and providing them with protection to carry out their plan.
PCs can find themselves challenged by a villain who is oddly powerful or resilient. It isn’t until they do serious damage to the villain that they’ll find out the source of their abilities. Even then they might not immediately identify the villains benefactors as being the Cybermen.
With their new cybernetic powers a villain might decide to turn on the Cybermen. It is likely that they’d foresee this betrayal (since they are such masters of it) and install failsafes that turn their agent into a puppet.
PCs could attempt to ally with the villain, convincing him that he can’t trust the Cybermen and that he will never truly be free while they still control him. If the villain does change sides would the PCs let him keep his cybernetics after the Cybermen are defeated?
The Sheriff’s declaration could provide a new origin for the Cybermen. He really could be a new breed of half man, half machine. In this case you can keep the original story but in this version the Sheriff survives his bath in gold.
Once free he is able to withdrawal and recover. He is shown to have some technical skills (his use of the computer display on his table) and so might know enough to be able to duplicate the technology and convert others.
He might encounter some of the droids from ‘Deep Breath’ and salvage their technology for this purpose. All he has to do is reverse engineer their harvesting of organic components.
This would create a new breed of Earth based Cybermen. They even have a reason to have a vulnerability to gold (imprinted trauma left over from their creators near-death experience).
You could also alter the origin of the Cybermen. Previously it has been indicated that it was the harsh condition caused by their planet leaving the solar system that drove them to begin the cybernetic conversion but what if the idea itself was provided by similar robots?
The mysterious robots could seed the idea of cybernetic conversion throughout space. This might be to ensure that organic and inorganic species eventually merge, thus preventing a future conflict.