Here is a continuation on my thoughts as to how each individual King would be viewed as a player and an NPC in the various Negaquests.
The Ohmu as a player: I see no way in which the Ohmu could be controlled as part of a quest, any quest starting from the toxic jungle would likely be from the perspective of one of the tribes that live there.
The Ohmu as an NPC: The toxic jungle is a ticking time bomb, it's eternally growing and uninhabitable for human life. While the Ohmu are not inherently malicious, and all destruction that they bring about is largely in self-defense or to protect other local bug life, they still pose a huge danger to everyone around them that cannot be ignored. Kings who are aware of the jungle are likely to attempt to find some means of containing or destroying it. It is worth noting that we are in a good position to potentially find another option, such as using a combination of the terraforming technology and bug control to relocate them to a new environment, perhaps on the moon, and allow the three states worth of land to be fixed up and made livable again.
DOR15 as a player: Doris seeks nothing short of complete and total global domination, very few of her hero units are there willingly, and those that are are completely psychopathic, evil, or both. a quest where you play as Doris is one that will have no peaceful relations, with the possible exception of the Master Control Program, but even then it would be doubtful. Her players would be forced to come up with multiple plans to infiltrate and slowly take over the territories of other kings, they are also in a better position than normal now that the Helping Hats have been introduced and they got a critical success.
DOR15 as a NPC: Doris is a clear and present threat to anyone and anything around her, any King that occupies physical space in the real world is a target for mind control by her. Any King that is aware of her existence should be making plans to destroy her by any means necessary, and honestly serves as a good practice for Bill.
Toffee as a player: Toffee manages to keep the majority of his real power hidden from Earth at all times, and very few appear to know anything about what he's truly capable of. His end goal is the complete and utter annihilation of the human race, due to their connection to Mewmans but is currently dealing with Phobos beforehand. There have been some surprising developments so far, and the players for a negaquest are likely either really happy or really really worried at the prospect of Demona becoming an ally.
Teffee as a NPC: Those who know the full extent of Toffee's power are rightfully worried and planning on eventually moving against him. Xanatos wasn't joking when he said that Toffee was one of the biggest threats to the human race. Anyone who isn't directly being manipulated by him, and is aware of his true purpose, is against him with only a few exceptions.
The MCP as a player: The MCP has no physical presence, but unparalleled power within the grid. In a quest where you play as him your primary concern would be to spread unseen power throughout the other King's territories and to prepare for them eventually learning of the grid's existence. That said, king candy and the cybugs also provide a threat that cannot be ignored. But seeking help in any way shape or form would be wildly out of character, and the players would likely not do so. They're probably also concerned about DoofOS and are likely the ones behind the cyber attacks, attempting to take control and assimilate any necessary programs from it. If they know about Doofenshmirtz having a copy of the laser that can allow transport into the grid they would likely be even more dedicated to ensuring that we could not pose a threat.
The MCP as a NPC: The MCP has a massive superiority complex, and unfortunately does have the skills to back much of its talk up, while most people do not know that it is in control of ENCOM any King who wishes to enter the tech market will invariably become its enemy. And Kings who learn of the existence of the grid will likely be tempted to try and overthrow the MCP, especially since despite its power it is vulnerable within the grid, especially with a user on the outside also taking action.
Yokai as a player: Yokai it's in a unique position compared to many of the other Kings, since he controls a conglomerate instead of a single corporation. He's highly secretive, nobody knows that he's even still alive, let alone that he runs the conglomerate, and is largely dominated by his desire for revenge. However some small things have shown that he might have done some reflection over the last several years and has somewhat grown. His players are largely dealing with superhero related shenanigans, and their recent infiltration by a cult that appears to worship Bill.
Yokai as a NPC: Yokai is by far one of the hardest Kings to approach, nobody even really knows that he's a King at all. One would likely only find out about him in truth if they either infiltrated and discovered on their own, or they managed to become a good and trusted enough ally for Yokai to reveal himself.
Waternoose as a player: His greatest strength is that he is located within a completely separate dimension then where the majority of the quest is taking place. He has no concerns for diplomatic relations or spreading his influence, all he wants to do is to continue to kidnap children to extract screams. Their greatest concern is their current alien infiltrator who is very, very, angry with them.
Waternoose as a NPC: All human Kings would automatically put him on their shit list upon learning about him, even some of the worst villains out there draw the line at children. the only Kings I can think of that likely would not have a problem with what he's doing would be Doom, Toffee, Bill, the MCP, DOR15, and Phobos. And even then they might still move against Waternoose for their own reasons.
AUTO as a player: AUTO is only concerned with capturing more humans and repairing his ship. I highly doubt there would be a quest where you play as him since it would be wildly out of character for him to poke around outside of the wasteland, leaving the rest of the setting completely untouched.
AUTO as a NPC: AUTO is a relatively contained threat, however the Midwest cannot fully recover while he's still there. Not to mention any King who managed to take him out would win the loyalty of all humans trapped within the ship, as well as a hefty boost relations with the US government. I do believe that any King that becomes aware of his existence will begin plotting his demise for their own reasons.
And for fun...
Doofenshmirtz as a player: We already know what that's like, it's us.
Doofenshmirtz as a NPC: