Excellent thread.
To me, the whole dimensional shifting is a little peculiar and it was introduced in an awkward way by Kishi, but given the magnitude of how powerful it was, it makes sense. My first thought was that they were simply different places on the Earth because I doubted that anybody could be powerful enough to stretch different areas of the universe. But then again, Kaguya is said to have more chakra than the Juubi. Now I'm going to use some hyperbole here but it might make some sense.
The recent data book asserts that one Truth Seeking Ball holds the capacity to obliterate your average forest. Now if we take into account that Kaguya's enormous Truth Seeking Ball is hundreds of thousands of times (maybe even millions) larger than the TSB's that we saw, we would literally have to multiple the damage done to one forest by let's say 700000. Making her scale quite nearly planetary/dimension level. With that being said and established, it doesn't seem as farfetched that Kaguya actually has the strength to transfer dimensions to completely different areas of the universe.
There's also some evidence from the manga and the lore behind the Ootsutsuki. They're alien and came from a different place (possibly planet) and they were able to survive on the moon, as the rest of the clan besides Hagoromo stayed on the Moon to guard the Gedo Mazou alongside Hamura. So if they have a link to places besides Earth, why wouldn't the strongest being in the entire manga? Remember when Kaguya was starting to utilize her Dimensional Shifting a lot, we saw a castle behind in the scenery and we know from The Last that there's a castle on the moon that the Ootsutsuki's placed there (Kaguya was regarded as a princess who came from a faraway land). That indicates that her shifting was actually shifting to completely different areas.
The other manga piece that sort of proves or gives more leeway to the thought is what Obito said about her technique. When Obito was using Kamui in the manga, he was able to travel vast distances around the entire world. So he essentially could travel around the entire world with Kamui. However, when referencing Kaguya's technique, merely synching with it was extremely hard for him and he called her technique "vast" and wondered what kind of monster she was being able to connect such vast and worlds apart dimensions with such ease and speed. So if someone who is basically earth level in his ability to travel with Kamui says that her technique is worlds apart in terms of distance apart in the dimensions, can it possibly be on the Earth, or should it be much larger and possibly multi-planet level? We might get some clarification in the Ashura/Indra, Kaguya epilogue prior to watching The Last.
