In ‘Hide’ the Doctor discovers the source of ghostly sightings is a pocket dimension, where a stranded time traveller is struggling to get home. The Doctor is not surprised at its existence, stating that they just happen.
He clarifies to Clara that it is not a parallel dimension. The very fact it is a pocket dimension indicates that it is ‘within’ our own dimension possibly existing as folded space like a tesseract. There does appear to be some form of ‘overlay’ as images from the pocket dimension are projected into our own.
The Doctor’s definition that it is a distorted echo could suggest that it attempts to mirror or copy our universe. Certainly the forest the Doctor finds himself in appears similar to those that could be found in the surrounding area.
Time within this particular pocket dimension moves much slower than in the real world. Several billion years pass in our dimension (the entire life of the planet) while the trapped time traveller, Hila Tacorian, has been there for only 3 minutes (although the Doctor is guessing).
It is probably safe to say that being inside a pocket dimension when it does collapse isn’t a good idea. The Doctor ends up rescuing the monsters, promising to take them somewhere safe.
It is unknown if the monsters encountered within are native to the pocket dimension (which would indicate enough time for some form of evolution to occur) or stranded like Hila.
We also don’t know if all pocket dimensions experience the same distortion in time. It could be that other dimensions have a faster flow of time or one that is equal to our own. We also can’t say how big they are. The one shown here appears to be at least large enough for an expanse of forest and open sky.
If the TARDIS entered the pocket dimension entropy would drain its power source, trapping it. If it was there when the dimension collapsed it would have to wait until the whole universe collapsed into quantum foam before it could get free.
Presumably during this time the TARDIS would survive but as part of the universe, unable to return to the vortex or normal space. The Doctor is unwilling to wait for this to happen so choices not to bring the TARDIS into the dimension (at least until his second rescue effort).
There is nothing to indicate that such pocket dimensions are unnatural or any exploration of what created it. It has existed since the beginning of Earth and will be there at its end. From what the Doctor says there could be plenty of other pocket dimensions on our world and others.
While time travellers could certainly blunder into them it is possible that other portals could come into existence or that random people might stumble in or out of them. When the barriers are thin those on either side might be able to see each other.
A pocket dimension is therefore something that can be encountered by the crew of a TARDIS or members of UNIT or Torchwood. You could even centre a campaign on a group who do nothing but seek out pocket dimensions and solve the problems they cause (something along the lines of ‘Sapphire and Steel’.)
Adventures involving pocket dimensions could require PCs to rescue those trapped within. They could be the source of horrible monsters or twisted reflections of our world. They might emit radiation or exert a strong gravitational pull threatening to swallow up everything around it. PCs could study them for scientific data, exploring their interior and trying to escape before they collapse.
The difference in the flow of time could allow PCs to use a pocket dimension as a time machine, along as they are only interested in going forward. If they become trapped in the past without a time machine this could be a clever way to skip forward until they are reunited with their TARDIS.
If they find a pocket dimension whose flow of time is backwards then they can also travel into the past. They need only work out the difference in the flow to time their entrance and exit and make the journey to a pocket dimension flowing forward to return to their present.
They could act as prisons, safe havens or used for storage. Having a secret base within a pocket dimension would certainly help conceal it from the outside world and the PCs enemies.
There is a clear example of a paradox in this episode in that the Doctor removes Hila from the dimension in 1974 yet earlier took photos of her far into the future, still trapped within. The Doctor states that these paradoxes resolve themselves, so pocket dimensions could act as a buff to prevent destructive energy from being released. PCs might find this useful if they absolutely have to create a paradox.
It is suggested that the experience of investigating this haunting is what brings Emma and Palmer together and that Hila is a distant descendant. In this case the pocket dimension is responsible for creating a paradox, in that Hila is only born because she is trapped within the pocket dimension. Without that her ancestors don’t get together.
This could be an aspect of a pocket dimensions, rewriting our reality with similar ontological paradoxes. They bring things into existence, creating loops of causality. One or two might not make a difference but numerous pocket dimensions could cause a Time Lord quite a headache.