The Mattress

Vamdemon's ambitions were a lot higher than Etemon's kingdom. His endgame, which also seems to be the Dark Masters' endgame, was to merge both worlds into a single mishmash reality that he would then govern as king.

What the Dark Masters have done to the Digital World with Spiral Mountain, Vamdemon was supposed to do to both worlds. Then, presumably, he would be the king... at their behest.

Like. According to monarchism lore, a king is supposed to derive his right to rule from the divine mandate of God. He gets to be in charge because divine forces beyond our ken have deemed him most fit to be our supreme leader.

Presumably, the "Four Heavenly Kings" of the Dark Masters would be the divine force whose mandate permits Vamdemon to rule the merged reality. I think that's the relationship they were going to have there.

Etemon, by contrast, does not seem to have any divine patron for his right to rule. Instead, his title as King of Digimon just seems to be Might Makes Right logic. He is the Strongest in the World therefore you gotta do what he tells you, the end.

F.O.W.L - They're a Team Rocket flavored criminal organization already. There's a boss, administrators, and even male and female grunts, and they've got their secret lair in a basement underneath an arcade! Put them in the world of Pokemon and they fit right in, exploiting Pokemon for self enrichment while seeking to prohibit Pokemon trainers from embarking on any more adventures with their own Pokemon.

The Forever Knights - And they're a Team Plasma flavored criminal organization! Actually, crossing them over with Pokemon would be more like Team Plasma from Gen 5 meets Ultra Beasts from Gen 7.

Afterlife Battefront - They may be morons, but there's a lot of them even beyond the leading crew we normally see, and they'd be an interesting paradox - shooting for high, Team Galactic tier goals while committing low, Team Skull-tier offenses.

The Apostles Of The Stars - I can see them operating like something out of the Orre region or a Ranger game, with Creed Diskenth being downright Volo-esque in his genocidal objectives.

The First Order - Unlike the original Galactic Empire, they're....well, not an empire, and are constantly occupying different places and launching different attacks in their efforts to become one. A group like that would be interesting to work with if they landed in the World of Pokemon. Plus we have the setup of Kylo Ren as the boss, Hux and other officers as the admins, and the Stormtroopers as the grunts.

Shinra Inc. - If we took out President Shinra and his board, had it just Rufus as the big boss, the Turks as admins, Hojo and his research team as the scientists, and Shinra Soldiers as grunts, we'd basically have Gen 1-2 era Team Rocket. I'm sure I'm not the first to have noticed this.

ACROSS - We never really got to see the whole scope of ACROSS as an organization beyond things like That Man as the boss, Key as an informant agent, shit that Space Butler was involved in that got Hyatt as a member, etc. So giving them a Poke' villain arc would be a nice way to explore that beyond our Jessie-James-Meowth-Giovanni flavored usual crew of Excel-Hyatt-Menchi-Lord Ilpalazzo.

The Beehive Gang - They're a mafia organization literally called "Beehive Gang." Change it to "Team Beehive" like has been done for all "Gang" evil teams of Pokemon and it just writes itself from there.

The Glittering Crux Brigade - Another total package with commanders, scientists, and grunts, plus we have Head being so much like Spinel.

The Black Knights - Would translate well into being a "movement"-based organization who commit terrorist operation for some percieved greater good, with Zero being like Cyrus and Ghetsis in terms of decieving his own followers in regards to his personal goals.

The Huntsclan - If Pokemon count as magical creatures, then these guys would be out to not only steal their powers, but kill them. This is also an evil group that I could see having a Team Rocket, Cipher, or Team Plasma style comeback after their initial outing since their uniforms and aesthetic style changed between seasons.

The HIVE - Another group that would be like villains from out of the Ranger games or Orre region. Heck, we could even tie them to Team Beehive the same way Cipher was to Team Snaggem.

The Fire Nation - Change the Fire Lord into a crime lord, his military officers into his executives, and their soldiers into grunts, with some special units like the Rough Rhinos, Zuko and his crew, Azula and her friends, etc., and this imperial threat becomes a more low key but still dangerous crime syndicate fit for endangering the Poke' World.

The Earth Empire - Same deal as the abovementioned Fire Nation.

The Equalists - Already the most Pokemon-esque bad guy group in the Avatarverse. An organized movement whose members believe themselves to be the righteous ones? A charismatic leader with a vision for the creation of a new world order but who's decieving his followers? A right-hand executive member? A chief mad scientist? Grunts who all dress alike? They check all boxes!

The Order Of The Red Lotus - Another group with plenty of members and operations we never got to see. This one would be most akin to the Pokemon Pinchers, with the four main bender leaders being the Societea equivalent.

The Eggman Empire - Put Dr. Robotnik/Eggman in charge of his own robotic-themed syndicate and there are a seemingly infinite number of ways that such a group and its arc can be structured and played out. And who hasn't crossed over Sonic and Pokemon?

The League Of Shadows - A far more secretive and seldom seen in the open evil team than what Pokemon is used to, but that's what would make it interesting to put in there. And since we had Gibeon with the Explorers, R'as Al Ghul being an ancient evil team leader still alive even in the modern day would not feel too out of place!