Been taking a look at Death Comes to Time for map reasons (since it's here we're introduced to Mt. Plutarch and the Lune Forest - the latter of which is also mentioned in Gallifrey: Time War, though it seems a lot of people misheard it as 'Loom Forest'). Got a couple of theories based on some details of the scenes with Ace:
1. There might be a connection between this timeline and The End of Time:
Ace recognises some of the constellations in the sky on Gallifrey from Earth. Casmus doesn't correct her. But this is ludicrous, the planets are at least 29000 ly apart, if not galaxies. Even if some of the stars can be seen from both Gallifrey and Earth, they would be oriented differently.
Now you could point out that 'constellation' is sometimes used differently in DW, such as for stars that are near eachother locally (this definition is used a lot actually), rather than angularly, or may even refer to stars that are temporally connected in some way. But that doesn't seem to make sense here. At best, maybe Ace is flat out wrong and Casmus is misunderstanding based on different definitions, or just ignoring the statement.
On the other hand, I think it has been suggested that Gallifrey's heavens behave differently (though I thought that was simply related to Gallifrey's changing flow of time?) so maybe this is some sort of alignment corresponding to "Ace's era"? One could probably link this in some way to the Younger World Story or Earth's caldera.
My suggestion, however, is this timeline might be related to Gallifrey being moved from the Time War into the Solar System in Rassilon's Final Sanction. Perhaps this is a universe where the Sanction was completed and reverberated back in time as it collapsed, as far as the Seventh Doctor's era, creating new branching timelines as it went. It would even help justify the Time Lords' powers in this universe, if they're in the process of ascending and becoming 'beings of consciousness alone' ala the Celestis.
In other words, the stars look like they do from Earth because Gallifrey is quite literally in Earth's place (though obviously not quite in Earth's present day).
2. "Saronite" explains why Gallifrey's oceans aren't as blue as Earth's.
A much less grandiose theory here, but in Ace's tests, she must cross the "Cavern of Infinite Death" without falling into the pool of saronite. When she does and believes she's been poisoned, Casmus explains:
ACE: But what about the poison?
CASMUS: Who said it was poisonous?
ACE: But ... but ... you said it...
CASMUS: I said you mustn't touch the saronite.
ACE: Well, what is saronite?
CASMUS: It is much like water, with one important exception.
ACE: What exception? What is it!
CASMUS: It is red.
Presumably the pool is water with some mineral 'saronite' mixed in. If 'saronite' is common enough across Gallifrey, it may tint the oceans enough, combined with the reddish glows of the Suns and the atmospheric tint, to result in the dark red-brown-purple-ish colour of the seas we tend to see from space.
Fun fact: I googled saronite to see if it was some real mineral, but all the results related to World of Warcraft, where it is the name of a mineral formed of the blood of 'Yogg-Saron'?
I don't know where saronite first appeared. If it came from Wrath of the Lich King, then the Doctor Who usage predates it. However I know Warcraft as a franchise is older than Death Comes to Time, so saronite might have appeared there earlier.
Either way, I guess Gallifrey's water is literally filled with the blood of an elder god. 🤷
