Purpose of This Thread
Decided to make this thread since people are split on Void Shiki/Akasha being omnipotent, multiversal, or Servant level (from Fate). While my goal is to clear up this debate in a single post and make this in a similar nature to a respect thread, I'd also like anyone to discuss their disagreements or opinions (if there are any).
Who Void Shiki Is
Void Shiki is the true third personality of Ryougi Shiki. Void Shiki initially only appears once in the Kara no Kyoukai epilogue, where she speaks to Kokuto Mikiya about who and what she is. She reveals that she is the original personality (the body) of Ryougi Shiki, and more importantly, reveals that she is connected to the Root (more commonly referred to as Akasha). Void Shiki states that she isn't just simply connected to the Root/Akasha, but IS the Root/Akasha, meaning Void Shiki is truly the physical manifestation of Akasha.
Who and What Akasha Is
Akasha is a metaphysical location within the Nasuverse, existing above all theories and dimensions in it entirely and acts as the force of the Nasuverse. Contrary to popular belief, Akasha is not just a location, but also an entity. Akasha became an entity once she gained self awareness as Void Shiki.
"Existing outside of time, it stores and archives information of all possibilities and events, past, present, and future, of the world. It is the place from where all souls, including those of Heroic Spirits recorded on the Throne of Heroes, originate from and to where they return after death." http://typemoon.wikia.com/wiki/Swirl_of_the_Root
Discussing Kinoko Nasu's Interview Statement On Void Shiki
A certain unexplained statement of Kinoko Nasu (the writer/creator of the Nasuverse) has split the community on Void Shiki's true power. Here is the interview statement... "Q: In the other Type-Moon works, who else besides Arc can fight against Servants?
A: If we're working on the condition of one on one, with an extremely average Noble Phantasm. Generally most of the 27 Ancestors, Kishima Kouma, Aozaki Aoko.
If it's just a defensive fight, but would still be a fight, then Ciel. Shiki (Rakkyo), Shiki (Tsukihime) are no match for Servants....but Ryougi Shiki (3rd personality) might be able to go as far as the Ciel class." http://typemoon.wikia.com/wiki/Shiki_Ryougi#References
This has naturally caused much discussion on whether to believe Nasu's unexplained interview statement, or believe his many in-lore statements, but what if I told you that it is extremely likely that Nasu is not talking about Void Shiki/Akasha, but rather the main Ryougi Shiki we see throughout the vast majority of the movie series? See, after the death of the male personality of Ryougi Shiki, the female personality drastically changed to the point Aozaki Touko theorized that the current version/state of Ryougi's female personality was an entirely new third personality; the idea of this being a third personality was shown way before Void Shiki was ever mentioned. "Touko understands the sense of detachment Shiki feels, and theorizes that the current Shiki is a third, new personality." http://typemoon.wikia.com/wiki/Shiki_Ryougi#Shiki This is heavily backed up by the fact that the Ryougi Shiki I am speaking of later proceeds to fight and defeat Nrvnqsr Chaos, a high ranking Dead Apostle Ancestor in Melty Blood, who Nasu states is Servant level (as shown in the previous quote of his above). The same Ryougi Shiki also defeated Araya Souren, who could fight against Aozaki Touko, who could fight against her sister Aozaki Aoko (who Nasu states is Servant level).
With all of this taken into account, it seems quite clear that Nasu was referring to third personality Ryougi as the one Touko considered the new third personality, and not her true third personality Void Shiki, as the Ryougi Shiki that Touko considers the third one has displayed the feats on the exact level Nasu was describing, while Void Shiki/Akasha have far more impressive statements. Now with that out of the way...
The Power of Akasha
Before I get into this, I just want to state that I will be referring to Void Shiki exclusively as Akasha from now on. For one, I prefer it and it is technically her true name, and two, I have a half broken keyboard and prefer to type the shorter name for convenience.
I would also like to state that I am NOT using VSBattles dimensional tierring, but the way dimensions are shown to work specifically in the Nasuverse.
Now there are two interpretations for Akasha's power; one being some form of multiversal, the other being possibly omnipotent.
Akasha being above the entire Nasuverse puts her way beyond baseline infinite multiversal. The Nasuverse has countless timelines and multiple infinite sized multiversal spaces in the Moon Cell of Fate/Extra, and has higher dimensions that are completely superior to lower dimensions, such as 6-D Avalon that is immune to anything from lower dimensions (where Saber/Arturia is), all the way up to the 8-D Moon Cell, where lower dimensional beings can't even exist in under normal circumstances (where CCC Gilgamesh is). Despite there only being eight confirmed dimensions in the Nasuverse, that has never been stated to be the limit, so Akasha could be anywhere from 9-D, to beyond infinite-D for all we know (since there is no confirmed limit), but I'll go with 9-D tor the sake of fairness.
Akasha also states this. “The spiral of origin is the primordial beginning of all things in this pattern of reality. All things pass through the great wheel, their natures and their histories—past, present, and future—are connected to it. It is thus a vast and empty place. It reflects what I am, in a way. I am connected to it, and I am a part of it as well. I am it. Which is why the greatest feats that mages can only dream of are allowed to me. I can change the very structure of elementary particles. I can transmute evolution itself, changing everything into something wholly different. All creation dances to the tune of magic and the melody of the arcane Art. I can bend the rules of this lie of a reality, this prison that keeps so many minds in sleep. I can break it as easily as a twig. I can remake this world. I can unmake it. I can make a new one, whole cloth.”
Kara no Kyoukai 8: Epilogue - page 227
Akasha is known as an apathetic entity that cares for nothing, so this is not mere arrogant boasting.
As for possibly omnipotent Akasha...
Her wish granting ability is described as all-powerful (which is another possible way of meaning omnipotent). "Shiki's Personal Skill Connection to the Root marks her as an all-powerful wish machine. Due to possessing this Skill, regular parameters become meaningless." http://typemoon.wikia.com/wiki/Saber_(Fate/Grand_Order_-_Shiki_Ryougi)#Abilities This is backed up by Akasha seemingly claiming to be omnipotent.
You may say "But Akasha is a mere Saber Servant in Fate/GO, so how is she omnipotent?" To that I say omnipotents can do whatever they want; implying she can't act as a mere Servant for the fun of it means she has a restriction (not omnipotent). Akasha seems to heavily imply she took on an imperfect form on purpose; whether she is referring to taking on a human form or willingly becoming a Servant is up for you to decide, but regardless, it changes nothing.
People may mention as well that Akasha is not omnipotent due to Roa and Wallachia from Tsukihime and Melty Blood surviving being absorbed into the Root, but nothing states or even implies Akasha is actually trying to absorb them; it is simply the true state of her existence. Maybe she purposely allowed loopholes for all we know.
Since we have dealt with Nasu's statement, Akasha stands anywhere from some form of multiversal to possibly omnipotent, but regardless, she is above everyone else in her verse entirely.