The Undertale Multiverse is home to Undertale, Deltarune, all their AUs, Out of Code Characters (Out!Codes), The Void(s), The Anti-Void(s), etc.
Idea 1[]
It looks like a black box with multiple video cameras with several different timelines. Cords are placed all around the room.
Idea 2[]
It is a long hallway with doors to each universe.
Idea 3[]
It is a collection of rooms and hallways, some small, some big, all different colors, different designs, rooms full of trees and grass or rooms full of darkness and light, and doors leading to each AU. Other occasional things along the way (planted pots, boarded up windows, etc.).
Idea 4[]
It is a series of disconnected bubbles that are in constant motion. Bubbles sometimes cross over one another, causing crossovers between the worlds. Movement within this space can be done by any method, a 'floor' or 'staircase' appears where a character needs it though they could swim, fly or walk in any direction and orientation through this space even if they normally lack these abilities. Passing into the world itself could be done by opening a door, hatch or tearing open the fabric of the world. The door or hatch restricts the entry point in the world where the tear does not. Structures can exist within this place provided matter can be stolen from physical world in order to do so. Characters lacking a physical form gain them while in this space.
Idea 5[]
It's a large void filled with misty-looking versions of the AUs, showing what the current situation is like in each. If characters live in the Multiverse outside of their AUs, then their home seems to drift around their AUs like a moon orbiting a planet.
Idea 6[]
It's a large room full of floating papers for each AU. You can float to one, and tapping one makes it turn into a door. Going through the door makes a hallway of doors with signs of the areas, like Hotland, Waterfall, or The Surface. Going through those doors will send you to that spot in the Underground, or to the Surface exit where you get a happy ending. Another version is that the papers don't turn into doors. You can go trout the paper, the paper acting as a portal to a random place in the AU. Also you can see a picture of what's happening on the paper of an AU.
Idea 7[]
A floating mass of islands, with random aspects from each AU.
Idea 8[]
It is a random rip in time that you can enter to go through to a random AU.
Idea 9[]
A big plaza with roads going off to each AU, branching out for their timelines.
Idea 10[]
A massive platform with some sort of hub in the middle. All AUs are connected via tunnels or wormholes, and each one has different accessible timelines, represented by bifurcations in the wormholes.
Idea 11[]
AUs are celestial clouds that come in different colors. Active AUs have a bright glow, Corrupted AUs are very glitchy, AUs that are nearly destroyed flicker and destroyed/abandoned AUs are completely dark, gloomy and empty. People who used to live in now destroyed/abandoned AUs turn into lost souls. Subs AUs circle around the AUs like moons circling around planets.
Idea 12[]
Very similar to idea 4, but the bubbles are not completely disjointed. Multiple takes on the same AU, combination AUs and their non-combined versions, or AUs that have the same creator or creators that work/have worked together tend to go near and through each other more often than other AUs.
Unlike idea 4 however, a floor or staircase to walk on doesn't appear, and, unless a surface to walk on is created/brought there, swimming/flying/floating is the only method of transportation outside of AUs and certain Antivoids. Antivoids appear as completely 2-dimensional portals from the outside, and can only be seen or passed through from one side, although some, like the Omega Timeline, have other methods of access as well, either from the moment of their creation, or implemented by the owner. In some cases, like the OT, the portal is camouflaged and/or closed off for security purposes.
Idea 13[]
Similar to idea 4, but the outer surface of the bubbles are made of a ridged, impassable material. The universes, while in constant motion, are specifically in constant motion around a central gravitational force, similair to planets in a solar system.
Idea 14[]
The multiverse is a giant bubble which keeps ever expanding. On it, there are sections connected through bridges or pathways. Those sections can group together and form AU clusters. The Doodle Sphere is used as a planetarium, showing the display of the AUs rather than the physical AU themselves. As well, the sections can split into various timelines, showing a display of them, represented as several ropes all connected to one spot. If a timeline is destroyed, the rope will be cut from it. The bridges CAN be destroyed, but it requires an intense power. As well, each AU has a Void on it, independently whether the Gaster event took place or not.
Idea 15[]
Some believed that the Multiverse itself was the manifestation of the Undertale/Deltarune Fandom as a whole; a place where all fanfictions, Fanons, Headcanons, and other Non-canon Undertale/Deltarune materials became functional AUs and OCs. This correlates to Idea Ω, where multiple interpretations of said multiverse coexist with one another.
Thus, the only way an AU, OC, or any form of Non-canon Undertale/Deltarune material would truly be erased is if they're forgotten in its entirety. This applies to all form of Undertale/Deltarune materials, both canon and fanon. This would, indefinitely, ties our real world to that of the Undertale/Deltarune Collective Multiverse; as if all IRL creators are unwittingly Blind Idiot Gods.
Idea 16[]
Idea 16 refers to the SCUDA Multiverse, which is similar to Idea 3, 9, or 10, with the "hub" location resembling a glorified Judgment Hall with several hallways. The AUs itself are only considered valid SCUDA AUs if they follow the SCUDA Guideline, which is the fundamental laws of the universe as defined by EUC that AUs must follow in order to prevent things from breaking as characters from multiple AUs interact. Outcodes exist in SCUDA and so do their associated locations. Outcodes can bring other AU characters to the "hub". The creator is not the creators of the AUs themselves but rather an unnamed deity even above Ink!Sans and Error!Sans who both creates and destroys.
Idea 17[]
Each universe is represented by a galaxy. Entering the galaxy will take you the universe it represents. The various galaxies are also split into certain groups - For example, Swap AUs are grouped together into a single place, close to eachother, but not enough to merge. If two galaxies merge, everything from both galaxies will now live together in one universe (for example, if the Underswap and Underfell galaxies merge, then the cast of both worlds will be put into the same universe, free to interact with their alternative counterparts).
Idea 18[]
It's by all means similar to Idea 2 - A long, endless hallway with doors to each universe. However, at the very beginning of the hallway, there is the doorway to the Undertale universe. Each universe is numbered - For example, Dreemurr Reborn is named D-001-U (Dimension-001-Undertale), given that it was the first AU. The Undertale universe is called D-000-U. If a universe is destroyed, it will destroy the door along with it, making an empty space between doors that can't be replaced. At the end of the hallway, there is a stairway that would lead to the Deltarune universes and all of its AUs. The Deltarune universe is called D-000-D (Dimension-000-Deltarune), and all of its AUs follow that style. Given how AUs keep getting made overtime, reaching the stairway would take longer and longer the more AUs are made, given how new doors are inserted between the stairway and the pre-existing doors.
Idea 19[]
Correlates to idea 15, the multiverse consist of fan contents found on the internet; and therefore all AUs are code base as each were based off of a site or story that were found online. The only way for those AUs to die is for it to be removed from the internet entirely. This would add in to the meta of it all, as destoryers could be hackers and malicious virises that take down websites and corrupt files of other works; while protectors were security system to defend against such threats.
Idea 20[]
The Universe is a series of floating doors that, when opened, will allow access to several glowing panels. These panels dictate where various parts of the AU should go, how they look, what they'll do, etc. These panels are implied to be able to be destroyed. The doors can also be destroyed, which destroys the AU. These doors are also implied to be repairable, to a certain degree.
Idea Ω[]
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The Alternate Multiverse theory
Essentially multiple multiverses of Undertale where events are similar in each one (such as Ink and Error existing in most), yet each is unique. Each multiverse could contain a different structure, such as all the ones listed above.
This idea is the only one considered 'canon' by the Undertale AU Wiki.