The twist might be that the good place sucks, it might be torture for the people there, unintended by the good place architects but still torture. Look at how annoyingly chipper Nicole Byers good place postal worker was, or how over the top PC and supportive the Committee is, imagine living among them or in a place they think is good, it could drive people nuts.
And of course there's too much of a good thing, imagine hearing your favorite songs every time you turn on the radio, getting your favorite food all the time, always friendly people who agree with you on everything and love every idea no matter what.
Our main four were great for each other because on paper they were awful for each other, the good place would NEVER put them together.
What if next week they run into an actual good place resident and that person screams "OMG, real people! Get me out of here, this is hell!"? That would be a better twist than cheating demons. They already effectively got rid of the bad place, the finale could be about getting rid of the good place too because so far they have only redesigned half of the afterlife.