I'm going to give my opinion, the problem with Wyll is Mizora. At some point, someone decided that what made him "cool" was Mizora, so instead of focusing on improving his writing, they leaned on more Mizora scenes. And she is not even well written, since for her to seem "intelligent", they dump down the writing of all the characters around her. And the one who suffers more is Wyll.
Wyll concept can be interesting. A warlock, with a fiend as a patron that is mostly a good guy who made a mistake. Or maybe someone who decides to pay a high price for what he thinks is right. But to make it work, they should have focused more on his story and tried to tie all its elements in a way that makes sense. So we have his father, who is a great duke and loves him. He grows up "privileged" and loved. Why someone like him would sell his soul? To become a hero? We are told that it is to save the city, but the story about Tiamat doesn't quite work because it is too incredible. In a way, it mirrors the "too convenient" that we can point out with Astarion when he tells us about when Cazador "saved" him by turning him into a spawn.
So let's say that Wyll grows up under a lot of pressure to live up to his father's name. In a political landscape where the wrong move might cause a lot of trouble for his beloved father. So he meets a beautiful lady and she is really nice to him, encourages him to become a hero. And the first time he follows his advice it goes really well, people start calling him the blade, blablabla. He knows there is a lot of "luck" (as in informal intervention that he is not aware of), but then again, he can't refuse to help people now. So he continues, and the nice, hot lady keeps encouraging him and setting him up to go on bigger and more dangerous adventures. Until he takes on something really big, with incredibly high stakes. And the trap closes and Mizora reveals herself and offers a deal at a time when young, naive Wyll feels he can't say no. So young Wyll agrees to a pact without really knowing what he is signing for. But then has several years to wisen up and learn about what that pact means. Except, he doesn't have a copy of the contract and has no clue about what is written.
That is when Tav meets him. He still needs to keep the facade of being a hero because of his father's political position. He wants to break the pact and Mizora has gotten used to treating him as a puppet with no say in things. But she makes a mistake and asks him to haunt Karlach under the wrong idea that she is heartless. But Karlach has an infernal engine as a heart. As a result, she is not heartless, Mizora has broken the pact and expects that Wyll won't realize. Enters Tav (or Wyll) as an origin doing the quest to: recover the contract, gather evidence of Mizora breaking the contract, and basically, go into an infernal trial to fuck Mizora for breaking the pact.
This is my take. I'm sure that there are other ways to do it. But the key is to never, ever, take agency from the player or dumb down characters to try to make another look intelligent. And if you want a character to be cool, you need to focus on that specific character writing, not add another character like Mizora or focus on aesthetics like the horns. Which looks cool, but doesn't compensate for the lack of nuance due to not enough scenes/writing to understand the character.
In my opinion, people would relate more to Wyll if we could see the pressure to live to a "hero's" image. With a father who is a hero and everyone expecting that he becames one too. And how things and fame can get to your head and make you lose perspective. Or a young mistake that you carry for years. Those are real life things that can make you connect. For me that is what is missing with Wyll. And I do like the character. At least when Mizora is not around. XD
To summarize why Mizora and Wyll are the way they are - horny players (this is for Mizora)
