In ‘The Name Of The Doctor’ in order to confer with parties in different locations and different time periods Lady Vastra engineers a dream conference. This allows her to speak with Jenny, Strax, Clara and River Song, until they are rudely interrupted.
Vastra reveals to a confused Clara that time travel is possible through dreams. Presumably she means that several minds can meet within this dream-state. If time travel is involved are Vastra, Jenny and Strax projecting their minds forward or are Clara and River Song having their minds sent backwards?
This dream realm could exist outside of time, in some form of astral plane. If there is chronology outside of normal events (marking the various stages of pre-time war, post-time-war, time crack, post reboot, etc) then those participating aren’t actually travelling in time, just converging in the same place.
The decor of the room where those assembled meet is designed by Vastra and the tea they drink is drawn from her memories. If this is a physical location then it could be a realm similar to the land of imagination.
If it is purely mental then this is simply a form of telepathy. Those involved only imagine that they are physically present in the room when that is just an implanted idea, where memories can be shared.
In either case River Song shows that she is able to impose her own on the setting, changing a cup of tea into a glass of champagne. If this is an issue then this should be a contest of presence.
Lady Vastra initiates this conference and presumably selects who will arrive. While this is relatively easy for those within the same time zone it is much trickier for those in the future (from Vastra’s perspective).
With Clara Vastra needs to place her in a trance-like state, possible through a soporific laced candle or letter. It could be that only in this unique state can Vastra ensnare Clara’s unwilling mind. Only when this event occurs will Clara arrive, avoiding the problems of summoning her before she’d met the Doctor.
River Song is more complicated since it would appear that this is post ‘Forest Of The Dead’ and technically dead. The River Song stored within the computer was a data ghost, a copy of her mind and not the real thing.
None the less River Song certainly acts like the real thing and is able to take part in the dream conference. This opens up the possibility of robots and computer intelligences being able to enter the dream realm.
This dream-realm is not impenetrable as the Great Intelligence and his Whisper Men are able to intrude. This makes sense for a being composed of mental energy but conceivably could be entered by other beings with strong presence.
A dream conference has lots of possibilities within your game, especially for player characters who spend lots of time apart. While players are able to exchange information across the table this can be a way to explain how their characters can do the same thing when not physical together.
On a basic level this can be done when player characters split up but becomes even more effective when they are in different time zones. Even if they only have the one time machine (which is the default assumption) PCs could be dropped off in different eras to investigate the same threat and still share their progress.
This can also facilitate campaign set ups where characters are permanently separated, each in a different era and place. They might never be able to physically meet each other (unless the time periods are within their natural life spans) but they can still keep in a touch.
They might have individual adventures which others can offer advice or do research in their own time. For example someone in an earlier era could investigate events or places that are impacting another character in a later time period while those in the future could research how things turn out or other events that relate to the matter at hand.
If you want to take things one step further this dream time-travel could be used to possess someone else. This allows player characters to physically interact in the past or the future while still blending in.
It could be a way to include NPCs that PCs have met without requiring them to physically travelling to see them. If they need the expertise or help of a NPC they’ve met before they could initiate a dream conference call.
You could require some effort on the PCs part to engineer a situation in which they are placed in a trance state. Of course the NPC only needs to enter the trance once for the PCs to access them at any other point in the future. You could also skip this requirement if you just want to include an NPC (maybe they naturally enter this state)
It is up to you to decide how easy and how common this ability is. Can anyone do this given the right circumstances or does it require the purchase of a special good trait?
Villains could use the same technique to communicate. PCs could wonder how they are organising themselves only to find out that they are gathering in their dreams. In order to find out what they are planning the PCs will need to find a way to enter the same trance state and enter the dream realm without being detected.
The villains could bring unwilling victims, including the PCs, into a dream realm. Vastra made her dream room pleasant but there is no reason a villain couldn’t create some hellish realm to torture or brainwash a victim.
If their dreams aren’t safe then that means that the PCs are very vulnerable. Staying awake is only a short-term solution as exhaustion will eventually take its toll. In order to stop it they’d need to seek out the dream summoner and stop them but since they could be any where and any when this is no easy task.