Basically the idea I had is for characters called the Origin Sanses.
Basically they are ancient, living incarnations and representations of various concepts in the Undertale fandom (not just a single universe or even multiverse, the entire fandom). They came into being long before the vast majority of AUs, and as they are more living concepts than actual individuals, they don't have a "true form" that the human mind can fully comprehend. They exist everywhere, at every point in time, on every layer of reality, but they can take a physical form at any place or point in time if they so choose.
The Origins are not technically Sanses. Each Origin is a living concept, and has an opposite concept that a different Origin embodies. The opposing Origins conflict with each other eternally across all of space and time. Their eternal stalemate creates balance in the multiverse and the fandom as a whole. They usually only take a physical form and directly appear to others when they plan to manipulate events to gain advantage over their opposite Origin, because each Origin wants to completely erase the concepts tied to their opposite (and by extension destroy the opposite Origin themselves), as they are the antithesis to the concepts that make up their own existence.
Origins also have a preference for the form they take, usually one that is most in line with what they represent. The main 4 I came up with are as follows: Origin!Nightmare (evil, wrongness, injustice, negativity, etc.), Origin!Dream (goodness, rightness, justice, positivity, etc.), Origin!Ink (creativity, motivation, creation, life, etc.), and Origin!Error (burnout, sloth, destruction, death, etc.).
The Origins' forms are reminiscent of their "original" counterparts at a glance, but with differences that make you able to tell them apart if you are paying attention. You would know Origin!Ink isn't actually Ink Sans if you know both characters.
"How do the Origins compare to the forms they take?" You might ask. Well, it's simple. Here's an example: Regular Dream and Nightmare are cheap knockoffs that only represent a miniscule fraction of what their Origin counterparts embody (guardians of Positivity and Negativity vs. the living concepts of Everything Good and Right and Everything Evil and Wrong). The same can be said for Ink and Error compared to their Origin counterparts.
Let me make it even simpler: Nightmare gets power from the negativity in the multiverse, but Origin!Nightmare is the concept of negativity itself, the reason for the negativity, the unfairness that the negativity doesn't affect everyone, and the motivation in people's hearts to cause more negativity. That is to say that Nightmare's power comes directly from Origin!Nightmare. Which is also to say that Origin!Nightmare is infinitely more powerful than Nightmare Sans (this applies to all other Origin Sanses compared to their regular counterparts).
As for their actual abilities, while they are infinitely powerful when doing things related to the concepts they embody, and are still nearly omnipotent when using their full power even outside of their concept's "domain", they usually "roleplay" as the physical form they are taking the shape of, mimicking (often more powerful) versions of their abilities. Origin!Ink, for example, will use his brush and ink, and Origin!Error will use his strings. Why? Because it's more entertaining for them that way. As they are locked in an eternal stalemate with their opposite Origins, they know that unleashing their full power will almost never matter, as their opposite will simply do the same. Thus, there is an unspoken agreement between them to simply "pretend" to not be as powerful as they actually are when taking physical form in the multiverse.
The Origins are also immortal as long as the concepts tied to their existence continue to exist. This is why the Origins "fight" each other by manipulating events throughout the multiverse and the fandom rather than battling each other with their own power. A concept has no body to hurt or destroy, it can only be "killed" when it no longer exists. However, the physical forms of the Origins, the "Sanses", can be destroyed, although it doesn't accomplish anything, because a new one can be made instantaneously with no effort.
Thanks for reading this far! What do you guys think? Any questions, comments, concerns? It's just a random idea I had a while back so it's not my most fleshed-out one.
(Edit: Made some slight changes to the wording here and there for clarity, and added a bit of reasoning for why the Origins are in eternal conflict with their opposite.)
(Edit 2: Added a general description of the Origins' power/abilities.)