Once in the tunnels in ‘Flatline’ the Boneless become three dimensional. They still wear the form of their victims. Their bodies have gaps and the move with a lumbering gait. Their name comes from the observation that they appear to be hollow inside. Their movement judders, with their form occasionally blurring. This indicates that they are still made up of much smaller components.
Why is it that they assume this new form? Certainly it shows further mastery of dimensions but if their goal is to eliminate the humans they are pursuing this is not the most effective way to do it. Indeed, they might have more success using stealth, as they did with George and Stan.
It could be that they want to inspire terror. They also might find it easier to navigate the 3D environment using 3D avatars. As we learn there are several layers to the train tunnels and anticipating that they might have to go up and down these human forms could help them.
If the Doctor has come to the wrong conclusion the Boneless could be trying to take on human form to better communicate with these 3D creatures. By appearing as them they might hope to improve their relationship.
It isn’t clear how many of the Boneless have made the trip into our world or if they can only manifest one avatar per victim. Certainly we don’t see multiple copies of the same victim in the horde but if they are only an imitation there is no reason why this should be the case.
Starting with Clara and Risby’s encounter with the Boneless at Mr Heath’s house the Boneless demonstrate an ability to reduce three dimensional inanimate objects into 2D. This appears to be their favoured tactic to confine their prey.
The fleeing group find that the Boneless have already flatted several of the doors that lead off from the tunnel. Did the Boneless anticipate that prey might try to escape here and plan ahead? This seems unlikely given that appears to be some distance from their usual hunting ground but could indicate that they are so highly intelligent that can foresee such circumstances.
It could be a sign that the Boneless are able to move very quickly. Once they realised the group were entering the tunnel they were able to get ahead of them, flatten the doors and then approaching from behind in 3D.
The isolation of the train tunnels could have been a future experiment. If the Boneless were indeed looking for a cross section of the population targeting a train carrying a mix of passengers could serve their purposes. Later we see that the Boneless are capable of compressing a train so it could be that their plan was to immobilise its engine and prevent the humans from escaping in the tunnels.
This conversion of 3D objects into flat 2D could be a means to gain the extra dimensions the Boneless need to become three dimensional. Stealing dimensions from the TARDIS appears to give them a huge boost in energy.
Without the presence of a TARDIS the Boneless might be much less powerful. Only by claiming more victims could they increase their power. This could give PCs more time to track and eliminate the Boneless, knowing that the more people who die the more powerful the Boneless would become.
The Boneless appear to be able to work together and pool their energy, as when they trying to bring Risby’s painting into 3D. Potentially this means that a group of well feed Boneless could convert whole buildings or cities into 2D.
What is apparent is the the Boneless can’t sense what dimension something could be. This is why Clara’s ruse works. Instead the extra dimensional energy the Boneless are pumping out goes into the nearest ‘drained’ 3D object, the TARDIS.
The Doctor creates the 2Dis to unflatten doors so must work on a similar principle. He never suggests that Clara use the device on the TARDIS so it either wouldn’t have worked or the idea never occurred to him. It is possible that such a device would need to drain three dimensions in order for it to restore dimensions to something else. If PCs find themselves in a similar situation they’d have to find a suitable donor to drain from before they could restore their own TARDIS.
The trick with the painting could expose another blind spot in the Boneless’ understanding of 3D space. There were examples of art and photos in previous victims homes. Did they believe that these were real things, compressed into two dimensions? Might they have imagined that humans found this practice acceptable or that this odd race stripped others of their dimensions and then displayed trophies of this act in their homes?
Having decided that the Boneless are monsters the Doctor banishes them back to their own dimension. He indicates that there is a good chance that they might not survive the trip back, as they shed their stolen dimensions.
The question is what happens next? Were the Boneless the only living organisms in their dimension and if none survived does that mean the Doctor carried out an act of genocide? If there were more of their species and none of their explorers returned would they mount a rescue mission or make another attempt years later? If some of the explorers survive what would they be able to communicate about their experiences and their encounter with the Doctor?
While it is doubtful that the universe at large becomes aware of this realm of the 2D (given that the Doctor believes this is the first time the theory has ever been proved) PCs could encounter the Boneless again.
They might try a more subtle approach or invade in mass, flattening wide areas. If this was all a horrible miscommunication they might be able to explain what happened in the future. This could have consequences for the Doctor, as he discovers that he acted too rashly.
Now that the existence of the 2D world has been proven PCs might want to explore there. If they can find a way to survive the transition this could be an exciting and weird new place to explore. It might also be a handy way to hide from threats in the 3D world.