In ‘The Five Doctors’ the 2nd Doctor tries to explain the contradictory history of Rassilon. He is generally accepted as a great figure but rumour indicates that he was also cruel, causing the Time Lords to rebel against him, locking him in his tower.
Confusingly the Death Zone was used before Rassilon yet the practice is known as the Game of Rassilon (although this could refer to the later attempts to get past his traps and reach the tower to acquire immortality).
The ghostly Rassilon who appears at the end of the story gives the impression that he was in some ways good since he had the wisdom to see that true immorality was a curse and that anyone who wanted it should be removed.
Yet when he is reborn in ‘The End of Time’ he is clearly evil. This is also how he is presented in the ‘Divergent Universe’ arc in the 8th Doctor audio adventures. Earlier than that the New Adventure books indicated that Rassilon used time travel to erase any rival races from history.
Something to consider is that we don’t have to necessarily restrict ourselves to one version being true. By now we’ve seen that it is possible for the universe to be revised and for future events to impact on the past.
The reason that the Dark Ages of Gallifrey have so many contradictions is that it experienced a period of frequent revisions. There could have been two or more versions of Rassilon, influencing events.
With the discovery of time travel it would stand to reason that the first thing a Gallifreyan would do would be to try and influence their own planets history. They might consider introducing time travel prior to its discovery.
For example the sacrifice of Omega could make time travel possible, allowing Rassilon to travel into the past and give the technology to earlier Gallifreyans (still powered by the Eye of Harmony in the ‘present’).
These proto-Time Lords abuse the technology to create the Death Zone. The decadent society they create might also lead to an incarnation of Rassilon who is equally evil. The original Rassilon could stop the games and try to reform society, confining his alternative self to the tower.
It is possible that there were several alterations until the perfect society was established. The knowledge of this process could be kept secret out of shame or the confusion about what happened could be because few remember the original history.
The forbidden knowledge of the black scrolls could contain this information. The consequences of this meddling could have far reaching consequences for the rest of the universe, if their history was also affected by these changes.
The 5th Doctor was keen to read the scrolls before they were destroyed. Might he have followed up on this later and learnt the truth? Could this be the secret knowledge he refers to in ‘The Rings of Akhaten’?
Travelling in the past of Gallifrey is rarely done but ‘Time Of The Doctor’ showed that it is possible. The Dark Times could another exception, allowing PCs to travel back and get involved in this difficult period.
They might not be the only visitors from the future. There could be Time Lords from many different futures. Futures where they rule the universe with an iron grip, a version where they are enlightened beings of peace, Time Lords who are vampires or cyborgs. There might be versions where different alien races become the Time Lords or win the Time War.
The PCs could be vital to ensuring that their version of Gallifrey comes into existence (maybe requiring them to put Time Locks in place to keep that version of events in place). This could be a lengthy process and until it is resolved the future (that is the period after the Dark Times) could be in flux.
This could be a good period for the Doctor to get involved in and might explain the 7th Doctor’s references to being involved with his peoples early history. You could have multiple incarnations of the Doctor all working together in this impossible time in history.
The Time Lords policy of non-interference could be a direct result of these problems (even if the public isn’t aware of the truth). They have learnt how fragile history is and do what they can to prevent any other potential futures coming into existence.
You don’t have to restrict yourself just to this moment in Doctor Who history. Post Time War a similar event could occur on other planets upon the discovery of time travel. This will probably resonate with the players best if you focus on Earth but other alien planets give you more possible futures to explore.
The revision of history and resulting contradictions can cause complications for the PCs. They might run into versions of themselves that have been changed by the universe rebooting or have to face the consequences of their alternative selves.
These alternative versions needn’t be evil. They might actually be better, kinder versions of the PCs from versions of the universe that are equally improved. Would the PCs be willing to sacrifice themselves for the greater good?
Another consequence of the universe being rewritten is the rumours and legends left behind. They could exist retroactively, so that the PCs were aware of them before they became involved in events.
This could mean that they are the legendary heroes they always inspired to be (becoming something of a self-fulfilling prophecy). This applies equally to their enemies, so they witness their opposition gaining the stature of stories that frightened them as children.
This kind of temporal damage and resulting complications can make the very nature of reality more exotic. Time travellers would have a unique view of the way things came to be that others would find difficult to comprehend.
The PCs will also have learnt an important lesson that there is no single truth. Anything is possible, anything could have happened. Their actions will not only determine what will happen but what did.