The accurate assessment version of the trope appears in Nobilis, where everyone of significance is equivalent in power to what a human would call a god. A great deal of the game's drama comes from dealing with this fact.
Which effectively makes him one, due to how the Immaterium works. Not counting the one thousand Psykers sacrificed daily to keep the Astronomican lit/the Emperor alive, he is slowly gaining more and more power as people believe in him and die in his name - hence, his position as the up-and-coming Fifth God of Chaos, the "God of Mankind". Then again, seeing as how he stopped a great fleet during the Age of Apostasy by creating a Warp Storm which is still raging five millennia later...
The Chaos worshippers who win the Gods' favours and turn into Daemon Princes, which are, according to the fluff, pretty damn powerful in and of themselves.
In the fourth edition of the game, this is one of the possible epic destinies awaiting characters that reach 30th level.
Cyric, from Forgotten Realms, goes this one step further. After becoming a deity, he creates a book, called the Cyrinishad, which will make anyone who reads it believe that Cyric is the most important being in the universe. He then reads it himself. He now believes that he is the most important being in the universe, and (for example) that if someone thwarts his plans, he is simply letting them do so.
It's worth noting to people that don't quite get the impact of this example that Cyric inherited the portfolios of THREE former greater deities in his ascension, and the Cyrinishad was STILL able to do this to him. It is also able to corrupt another deity, Mask, before the book and its author are sent into exile by the deity of knowledge. And that's without even bothering to mention the fact that Cyric is obviously evil and thus the book is made with pages of human skin.
Even back in the old days of the boxed sets, high-level characters had the option of going on a quest for and potentially achieving 'Immortality' — godhood in all but name. Many (if perhaps not all) Immortals of the Known World (Mystara) were implied to have gained their status in precisely this fashion.
After describing the quest for immortality as a game-ender, TSR released a follow-up book giving rules for playing as an Immortal, complete with your own mini-universe and godly powers. And that book described rules for how you could transcend the entire Immortal hierarchy to become an "Old One" before giving up and declaring A Winner Is You if you get that far.
The Great Curse is instant A God Am I inspiration for most Celestials. Sidereals are the most prone to the pride overload, as their Curse is Hubris (others get possessed by their Virtues occasionally). This does mostly fall under "accurate self-assessment": most Exalts could take down a minor god fairly easily, and a major one if they team up or get their Essence up high enough.
The Ebon Dragon's ultimate goal is to usurp the place of the shinmanote physical laws, only weirder that underpins existence, essentially making his own nature the foundation of all reality. Since the Ebon Dragon has exactly no redeeming features, this would be a very bad thing.
In Scion a player character ascends to godhood when they reach Legend 9
A common ailment in Magic: The Gathering, mainly for Planeswalkers and Yawgmoth. (Since Yawgmoth's Dragon, Gix, had his own priests, it's likely he had a bad case of this too, possibly hybridised with Caligula Syndrome.)
Changeling: The Lost features an Eldritch Entitlement (a very powerful, very old noble order) known as the Lost Pantheon, whose members believe that the ministrations of The Fair Folk have turned them into something more powerful and more primal, an aspect of forgotten divinity. One of their Entitlement benefits is the ability to draw Glamour from worship by mortals. Most notably, one of the qualifications for the Entitlement is that the changeling's Clarity must be 6 or less - meaning they're out of touch with reality and close to thinking like a True Fae.
Likewise, this is why the present of Mage: The Awakening is so crap. Mages, considering themselves gods above men, build the Celestial Ladder to climb to the Supernal Realms. The first ones up, the Exarchs, then proceeded to kick the Ladder out from under them, making magic much harder for people on Earth and causing the Abyss to come into existence. They still consider themselves gods and stewards of reality, and believe that only their worshippers should get the sweet, sweet candy that is magic.
Some humans in KULT. They're actually right, but it's far from easy to walk that way.