It's clear that no potential ending to Game of Thrones would satisfy everyone. The series had grown too complex to make it easy for any conclusion to satisfy every important loose end. That's likely one of the main reasons author George R.R. Martin has been in no hurry to write his own conclusion for the book series. While the TV series' final season had many significant problems, even the best possible version of the ending would attract complaints.
Of course, there is one thing that would have easily made the final season better... and no, we're not talking about Azor Ahai! We're talking about confirming the fan theory that Varys was, in fact, a merman.
This insane yet surprisingly convincing theory dates back at least to a Something Awful forum discussion when Season 3 was airing. The post was then copied onto the r/Game of Thrones subreddit on June 22, 2013. From there, it went viral. It was the sort of theory just ridiculous enough to be hilarious, yet detailed and well enough thought out to seem like something that could actually happen.
The evidence is laid out in great detail in the post itself, using details from both the books and the TV series. Varys is constantly associated with water, and has an odd smile possibly trying to conceal sharp teeth. He doesn't have a bed in his quarters, which could mean he sleeps in the sea, and tells Tyrion he'd be surprised by the results if thrown overboard a ship. Everyone thinks he's a eunuch, but if he's a merman, he'd simply not have human genitals at all. Mandon Moore and Illyrio are also implicated as mermen in this theory, with Moore having "fish-like" eyes and Illyrio sharing water associations with Varys.
Merlings are a concrete part of Westeros' mythology. House Manderly has one on its crest, and Varys' frenemy Littlefinger has a ship called "The Merling King." Brienne is warned about "squishers," human-eating fish people capable of walking on two feet. So much of Game of Thrones is about creatures of myth thought to be imaginary (the White Walkers) or extinct (the dragons) becoming real and dangerous threats. Revealing merlings have been shaping Westeros all along would fit thematically with the main drive of the story.
The "Varys is a merman" theory continued to pick up steam as the show progressed, offering an explanation for why Varys could travel so quickly back and forth between Westeros and Essos. Of course, coming up with consistent travel distances became an increasing issue for all characters in the show by the final two seasons, so this theory became less of an easy fix for a single inconsistency.
Still, it remained a popular enough theory that actor Conleth Hill ended up addressing it in an interview with Nerdist's Alicia Lutes in 2017. Hill, for his part, was completely unconvinced by the theory, saying he didn't understand where it came from and that those who believed it were doing "too much marijuana."
So Varys' actor debunked the theory, but what do actors know? They don't write the series, and there was still another season left to surprise everyone with official confirmation of the theory.
For some, it appeared as if Season 8 Episode 4 was offering further evidence. Some viewers theorized that Tyrion Lannister was rescued by Mer-Varys after falling overboard during the Iron Fleet's sea attack. Whether these theories were joking, serious or, as Hill believes, completely stoned is uncertain.
Of course, Daenerys had Varys burned to death one episode later, and any hopes of mer-people taking over Westeros died with it. However, given the books held more evidence than the TV show, it's not impossible we learn the truth about Varys when George R.R. Martin eventually finishes his last two books.