These two questions are related, so I'm grouping them together.
Anyway: Good question, yeah. You could conceivably have a "High 1-A+" tier that essentially serves as a capstone for the entire Tiering System and the highest you can possibly go before reaching Tier 0. In this case, it could be formulated as being just the collection of all possibilities, where Tier 0 is the pure actuality that serves as the grounding and foundation of all these possibilities, while not being, itself, one of them. Basically the set of all possible things that the Tier 0 can bring into existence, which, obviously, is a composited thing that excludes the Tier 0 itself.
You could associate this with a Maya/Brahman dichotomy, where, if the Monad is the dreamer, the High 1-A+ character/structure is essentially the framework of its dreaming itself, which contains all things that the Monad can dream. You could also say that, while High 1-A+ is the set of all possible combinations of characters and letters and sentences, Tier 0 is the thing outside of that (See
here for an example of such a thing), which brings all of it into existence or at least serves as the basis of it.
A can imagine a few characters that'd qualify for that tier, and it'd have some interesting properties that differ it from lower tiers. For example, you can't be above High 1-A+ without being Tier 0. And you can also have only one High 1-A+ per verse (You can't exactly have multiple collections of all possibility). Overall good food for thought.