What can be bigger than Absolute Everything or it is the biggest concept, the one-above-all concept, which encompasses all things without exception (tangible, untangible, things that we can imagine and that we can't)? Is it really that «Absolute Everything» means Absolute Everything or it's just a conception which is limited by our mind and there are exist something bigger and greater than it?
You might want to ask yourself whether 'absolutely everything' is even a concept, or if it is simply a placeholder for one.
Kant made this objection that 'exists' is not really a property of things: If 'exists' is a property of things, then 'not exists' is a property of things. But although 'not exists' does seem to actually apply to some things, like "the integers between 6 and 7", those things are not things, given their objective absence. What 'not exists' actually applies to are descriptions or criteria.
Therefore 'exists' is a property of criteria and not of things. If a set of criteria actually describes something, then that thing exists. If the set contains an internal contradiction or some other sort of impossibility, then 'that thing' does not exist. But then, of course that thing is not a thing.
Your concept of 'absolutely everything' similarly does not apply to things, but only to definitions of things. We can decide that purple unicorns are included in 'absolutely everything', or that they are not depending upon how absolute a meaning 'absolutely' has in our current mood. But that decision is not about purple unicorns, it is about what kind of concept we consider the notion of "purple unicorns" to be. It is really a property of concepts, and not a basic concept itself, applicable to things in general.
The different notions of 'nothingness' have this same nature. We cannot conceive of Berkeley's set of "all things of which we have no concept". We can't start listing them, as that would involve imagining them, which is impossible until we have the concept of them. We can only list internally inconsistent or otherwise defective descriptions. Likewise, we cannot actually have the concept of 'absolutely everything', we can only have a pointer to the empty set of restrictive criteria -- the property that says 'yes' to anything it is given to judge. Actually conceiving of absolutely everything is obviously beyond us as limited creatures.
From a linguistic approach: every-thing just relates to every "thing" which is an object that one need not, cannot, or does not wish to give a specific name to (my dictionary says so). My dictionary defines "object" as anything visible or tangible and relatively stable in form.
So, "everything" means: Every material construct that is visible or tangible and relatively stable in form. -> Everything is not the word that describes every perceivable circumstance (objects, feelings, thoughts, abstract concepts, phenomenons...)
EDIT: My native language (German) translates "everything" into "alles" which is not a combined word from the translations of "every" and "thing" but is a concept which actually means what one intuitively understands when hearing "everything". So it's really just a definition/linguistic problem
EDIT2: To add "absolutely" to the "everything" doesn't change it because the former means: "Not limited or restricted in any way" and every: "The highest possible degree of anything" = The lowest degree of limitation = not limited = absolutely (hopefully no linguist every finds my address for this lol)
If by 'Absolutely Everything' you mean the set of all possible entities, then no, there are no possible entities not in that set, by definition. If you mean the set of all existing entities, then, unless you think that all possible entities exist or that only existing items deserve entity status, then there would be a larger set.
There is a chance that the 'Absolutely Everything' set is impossible if it turns out to be the set of all sets. The proposal of a set of all sets leads to contradictions. I guess you could include all sets of finite cardinality.
Yeah, I bet it's the set of all sets of finite cardinality, can anyone go bigger without a contradiction?