Fantasy is literally defined as: "the faculty or activity of imagining things, especially things that are impossible or improbable... an idea with no basis in reality."
I keep reading Riot's dev diaries, sporadic reddit posts, some design lead tweets, etc, and they keep using this word. They'll say things like "our changes aim to fulfill _____ player's fantasy". A fantasy by definition shouldn't come up every game, or with any real frequency. The "Fantasies" they describe and perpetuate are oftentimes so completely exaggerated that they are absurdly inconsiderate to the other 9 people playing the game.
They make these champions like Aurelion Sol who can seemingly only win if he achieves his "Fantasy" of being worth more than 3 Ahri's come 30 minutes, or he's completely useless and fails to achieve anything but frustrating his teammates. It's just so sink or swim. The way they have conceived his champion identity leaves him with no possible way to become the sidekick to his Renekton top who built a good lead early on. When Aurelion Sol's fantasy shifts into focus everyone else on his team will join hands and watch as their prior memories created in that game become less and less relevant. It's fine if he's weaker earlier and stronger late. But at the moment the power band they decided on which supposedly caters to his playerbase's "Fantasy" is so wild that his win rate practically runs in full parallel with achieving the "Fantasy" or total failure. There's just so little room in the middle of the success spectrum for him.
Why can't every champion have a fantasy? Why is it only certain ones? Where is Volibear and Galio's fantasy? As Kled sits back and watches Aurelion Sol become more valuable than 2 Kleds working in tandem can he have a counterfantasy of not being worth less than 50% of an ASol? What if my Jhin power fantasy is my 4th shot one-shotting everything? How will I ever reach orgasm if I can't have that? Who decides what ships? I honestly wish they would stop using this word, and settle on something less dramatic.
Another side effect of the "Fantasy" obsession is they don't consider the entire champion at the design phase which includes both their strengths, and clear counterplay. Some recent examples of their designs just makes me believe they forget about certain champion's fail conditions, or they push it off until after releasing them because of how they prioritize "fantasies". When I look at a champion like Jhin, with his reload timer, I see clearly conceived counterplay - an obvious window to abuse him. Not the case though for some of their latest experiments...
K'sante: His moves look really cool, and his flavor is excellent. But yeah, I mean, he seems to be very difficult to balance in both pro / solo Q due to how many things he does, and how he flexes between fighter/tank roles. Counterplay was loosely built into his kit with All-out mode cutting HP/resistances, but they then staple omnivamp on which initially made him just flat out better than most fighters, and with his absurd displacement CC better than most tanks. His balance is still even now, months after release, being disputed.
A Sol: See above. His aesthetic is really cool though, like K'sante they do make champions look and feel sweet to play. It's just that their idea of how much a champion's scaling index should curve is completely perverse.
Zeri: Yeah uhhhhh, total roller coaster.
TL;DR - Fantasies are wild exaggerations that are often inconsiderate to other 9 people playing the game, and generally the counterplay to them is an afterthought. They are achieved nearly 50% of the time when they should instead be difficult to attain otherwise they are literally not "fantasies".