Warning! Spoilers ahead for My Hero Academia chapter 336!
Mangaka Kohei Horikoshi of My Hero Academia inevitably chose correctly by making Yuga Aoyama the actual traitor instead of Invisible Girl despite what he originally teased. But ultimately, the desired effect was doomed to fall flat because of how ineffectively the series depicts the students of class 1-A.
It's obvious by how Horikoshi presented the cliffhanger at the end of chapter 335 that he wanted the traitor reveal to be a big deal. But it wasn't because he mistakenly chose from the beginning that he would put the focus on every class 1-A student. This could have worked if Horikoshi spent more time on each character, but then that would have meant that the betrayal couldn't have happened until much later - if he wanted to make a larger impact. But since he obviously didn't, the only other choices, besides Deku, would have been a slew of two-dimensional characters except for the big three - Bakugo Katsuki, Uraraka Ochaco and Shoto Todoroki - and, possibly, Tenya Ida.
Horikoshi has made many valiant attempts to put the other students of class 1-A in the limelight at various instances throughout the manga, usually through class competitions and battles, but every time, the focus has always inevitably fallen on either Deku or the big three, and sometimes Tenya. Case in point, the only reason why the annual school festival didn't end up as filler like it started out as was because one of these characters made a surprising turn. Bakugo actually fought hard to ensure class 1-A played as a band together. This was a major bout of character development for him since being in a band requires teamwork, and Bakugo had always preferred working alone until that time. The entire experience also made Bakugo a better hero as a result.
Why couldn't the traitor have been one of the big three?
Other than Deku, the only students whose betrayal would have shaken fans to their very core would have been Bakugo Katsuki, Uraraka Ochaco, Shoto Todoroki, and, possibly, Tenya Ida. But even making any of these characters the traitor would have presented some problems. Making Shoto evil would have been overkill since his brother Toya had already assumed the villainous identity of Dabi. Uraraka would have ironically been the perfect choice because she recently proved herself to be a more capable hero than Deku. But her defection would have undoubtedly sparked outrage among fans who have been rooting for her and Deku to get together,
Bakugo could never in a million years be the traitor because of his recent confession and apology to Deku. Many fans might view that as a moment when Bakugo finally redeemed himself, but in truth, it just proved how far gone he actually is from redemption. Based on how badly he's treated Deku over the years, saying "sorry" shouldn't cut it. His betrayal would have minimized this dynamic considerably and alleviated his suffering - even if he chose to align himself with All For One because of his guilt.
Why not Tenya?
The best and only possible choice would have been Tenya for the sole reason that Horikoshi has been favoring him lately. But similar to Ochaco Uraraka, his betrayal would have been unexpected because of how much effort he's recently put into helping Deku.
The most recent example of this occurred when class 1-A attempted to stop Deku from facing All For One alone. All of his classmates shared a heart-wrenching story of how he helped them. But all of these confessions never went anywhere because they were all overshadowed by Tenya when he surprisingly served as class 1-A's trump card. This outcome proved on numerous levels why Horikoshi should have chosen to focus on fewer class 1-A students from the start. Horikoshi demonstrated that he only needed Tenya to try and save Deku to achieve the overall desired effect. He also proved that heroes can be contenders for "opportunities" like traitorous backstabbers just because they receive more time in the spotlight even if it doesn't even fit their character.
Why not Yuga Aoyama or Invisible Girl?
Yuga was a better choice than Invisible Girl for a number of reasons, but Horikoshi didn't get the shock value he wanted because he teased that Invisible Girl betrayed hero society instead. That attempt failed for the same reason why all of Deku's other classmates would have, too: A lack of time on the page.
In the end, Yuga made more sense but only because readers learned the reason why he betrayed hero society in the same chapter that revealed he was the traitor (chapter 336). So even if Horikoshi teased him instead of Invisible Girl, readers wouldn't have cared until they started reading the next installment. Within those pages, readers discovered that Yuga was born quirkless. This not only connects him with Deku on a deep level but also makes him the very personification of irony incarnate. Yuga has always been full of himself, mostly on account of his Navel Beam quirk, which he wasn't even born with. He received it from the villain All For One. So the entire identity he eventually espoused only served as a massive cover-up of a lie. Even more ironically, Yuga's parents chose to make the deal with All For One because they didn't want their son to be shunned by society. Yuga also expressed fear of being alone, which undoubtedly helped his parents' eventual decision. And yet, the way in which he would later flamboyantly brag about Naval Beam, a quirk that was supposed to be what made society embrace him, actually isolated him.
Additionally, even though there were a few moments earlier on in the manga when readers began to suspect something was seriously off about Yuga, they transpired almost 200 chapters ago. During that time, Yuga began to exhibit signs of an obsessive nature with Deku that eventually culminated in him stalking and staring at Deku creepily while he slept. Eventually, Yuga confessed that his actions were solely based on the strong sense of kinship he shared with Deku since both of them possessed quirks that caused them harm. While it's clear now that Yuga was only telling half of why he felt connected with him, all of this is diluted because of how long ago the event transpired and since Horikoshi wrote himself in a corner by failing to continue building upon their relationship in a meaningful way. Instead, he took the time to do the exact same thing with Deku and all his other classmates.
As a result, Yuga and almost every other student in class 1-A are now just semi-fleshed out to the point where it doesn't really matter if any of them is a traitor. It's possible that Horikoshi chose to tease readers with Invisible Girl for this very reason to shock them. They were shocked, but not for the reason Horikoshi intended. They were shocked that he would tease something that very few readers would care about. They were shocked because Invisible Girl hasn't played any significant role in My Hero Academia in a long time, nor have her classmates like Yuga Aoyama, for that matter. Now, they're just left wondering why he would choose to capitalize on the concept of a traitor within class 1-A in the first place when there are so many more important things to cover in this final arc.