What new Monster Type can be in Yu-Gi-Oh! next?

As of Yu-Gi-Oh! SEVENS! we have no upcoming monster type like ZEXAL or monster summoning method like GX, which means the focus will on underused monster types. But since there will inevitably new monster types, we will discuss possibilities. We will start with last year's (2019) Reddit thread, and move on to extracanonical types, and then we will speculate on the existing types.


https://old.reddit.com/r/yugioh/comments/c16pc9/if_you_could_make_a_new_monster_type_what_would/
We will discuss the top ideas from this thread:

1. Frost: This idea is proposed because all of Ice monsters tend to end up in Aqua, such as Ice Barriers and Mobius the Frost Monarch. This idea would help an analogue for the Ice-type in Pokémon. The problem with this idea is that Aqua is one of the least populous monster types in the game, and even though they passes the 200 mark, splitting Aqua-type doesn't make sense for this time.
2. Spiritual: Though there is already Spirit monsters that fit this bill, they are based on Shinto entities, not spiritualism as a whole. This type would also work as a better analogue for Ghost-type in Pokémon. The problem with this idea is that most Yokai and Spirits are classified under Zombie and rarely Fiend, and Zombies are not particularly common as a type, they only seem that way because most Zombies are non-archetypal. Though idea works as the third DARK attribute representative for Fiend and Zombies.
3. Spacian: Similar to the Cosmic-type idea, this idea would cover space related monsters. The problem is, most Space related monsters are classified under Reptiles, such as Aliens and Worms, which is one of the least common types, more so than Aqua and Zombie. So this seems unlikely too.
4. Microbe/Virus: There really isn't much Virus monsters for this to work with.
5. Gust: Based on the idea of four elements, this would cover the elemental need for WIND. However, since 5D's the Thunder-type has been used as the secondary elemental type for WIND, even though it also used for LIGHT. So while this is very conceptually sound, because of the lack Thunder-types I wouldn't go for it. But this is probably the most immediately necessary of the pack due to lack of WIND monsters besides Winged-Beasts, which are fairly common.
6. Poison: Similar to Frost, this idea is largely covered with Plant and Reptiles, both of which are extremely limited. 
7. Elemental: While decent, since there already Rock, Pyro, Thunder and Aqua, this idea would only work for WIND, DARK and LIGHT monsters, since WIND and LIGHT share Thunder and DARK uses Spellcasters and Fiends as Elementals, both of which are exceptionally large.
8. Rogue/Thief: While my least popular idea here, this is probably the likeliest idea, since Warriors do not have a side type besides Beast-Warriors, which got plenty of support recently.
9. Cyborg: Similar to above, this idea would likely work as a mid-point for Machines and Warriors similar to how Machine and Spellcasters have Psychics.
10. Mythic: Based on mythical beasts. Since most Beasts are based on mythological creatures and we have plenty of archetypes based on folklore like Nordic Beasts and Danger!s, this archetype would need to be based on Occult avatars, though it seems Megalith archetype has made Rocks open for this idea.
11. Abomination: Unlike the idea listed here, this idea can be used by hybrid monsters, even though most of Beasts are also hybrids. Nonetheless, this idea can work for artifically created monsters and alchemical beings.
12. Drake: Suggested as the third Dragon analogue, it would be actually be the fifth, as Sea Serpent and Dinosaurs are also "dragons" due to their Japanese names. This idea can work for WIND based Dragons like Dragunities.

For the extracanonical types:

1. Yokai: From the GX manga, most Yokai monsters in the GX manga and elsewhere like Mayakashi archetype has been put in the Zombie-type, so while conceptually sound, this idea is already used by a underdeveloped.
2. Immortal: From the Duelist of Roses, this type's monsters are largely Machine in the real game, which already has two spin-offs, the fairly large Cyberse and less large Psychic. So another Machine spin-off would be overkill for the time being.
3. Sea Beast: From the Toei beta cards. Since Aqua, Fish and Sea Serpent already cover this, this idea should only be used if a Frost type were to exist.
4. White Magic/Dragon Magic/Dark Magic: From the Toei beta cards. Unless they are magic Elementals, Spellcasters and Psychic already cover them for the time being.

For splitting existing types, we already have Psychic for Machine and Spellcasters, Cyberse for Machines and Wyrms for Dragons. The overall trend for new types seem to be based on large main types, so the other major types that don't have large factions are Warrior, Fiend, Fairy and Beasts. We already discussed Warriors for Rogue, so we will focus on the others.

1. Avatar/Holy Beast: Already discussed in the main thread, but the idea I have is closer to Avatars in Megami Tensei, which are the animal avatars of deities. Even though Divine-Beast exists, that only exists for Egyptian Gods, and this would be more of a mixture of Fiend-Fairies and Beasts.
2. Magic Warriors: Obviously these would be the mid-point for Warriors and Spellcasters. I like this idea less since there are already Beast-Warriors and Psychics, and plenty of Warrior/Spellcasters in this idea.
3. Demigod: Similar to Genma race in Megami Tensei, these would be in-betweens of Warriors and Fairies/Fiends. Could also be named Meta, or Famed, also similar Megaten races.
4. Ogre/Orc/Oni: While this idea is technically covered by both Fiends and Zombies(due to cards like Mezuki), it can work still as a physical variant of Fiends, as most modern Fiends are more elementally oriented.

Edit (26.07.2021): SEVENS has recently introduced the Cyborg-Type for Rush Duels:

https://ygorganization.com/mynameisnthugoitsfusion/

We don't know if it will be introduced to the OCG/TCG.

Edit (18.08.2021): https://ygorganization.com/starstarstarstar/

SEVENS has also introduced Magical Knight and High Dragon types for Rush Duels.

Edit (21.09.2021): https://ygorganization.com/taiyounomannakahe/

SEVENS has also introduced Celestial Warrior type for Rush Duels.

Edit 2 (21.09.2021): https://ygorganization.com/youdontpickthecitypoplifethecitypoplifepicksyou/

SEVENS has also introduced Omega Psychic type for Rush Duels.

Edit 3 (18.12.2021): https://ygorganization.com/cardgamesinspace/

The new Rush Duel series, Go Rush!!, has introduced the Galaxy-Type.

Edit 4 (22.03.2023): https://ygorganization.com/backtobasicsforawholenewtype/

Illusion Magic from the Toei beta cards has been introduced to the OCG/TCG.