I'm a huge fan of Black Clover, and for me, the perfect ending would be Asta becoming the Wizard King. But, can we talk about the possibility of that not happening? There's a small chance this might not come true (I don’t want it to happen, but it's a possibility we need to consider).
You might ask why I think that's a possibility, so here's my reasoning (maybe what I say doesn't make much sense, but bear with me). In terms of hierarchy, Asta is still just a regular knight. Has he saved the kingdom multiple times? Yes. Is he one of the most powerful characters among the Magic Knights? Absolutely. But despite all that, he remains a simple knight.
I believe people underestimate the importance of hierarchy, but it's crucial. It teaches responsibility, organization, and leadership. When you become a vice-captain, you start to feel some of the responsibilities that a captain has, but not to the same extent. You begin learning how to lead. As a captain, you not only have to manage people, money, missions, and everyday tasks, but also motivate, protect, and be someone people want to follow. Yami, for instance, might sometimes seem like he's not the best captain, but in my opinion, he's an incredible leader. He always pushes his squad to surpass their limits (he's not afraid of his subordinates evolving, but rather encourages them to improve every day because he knows they can). He also protects them when necessary—interrupting Asta's trial is a great example of that. He's just a guy with a small stomach problem who likes to spend some money.
Another point is that if you want to be the Wizard King, it’s not enough to be the most powerful; you have to prove you’re a capable leader and will be a great emperor. Power is important, but you need to show everyone you have the leadership skills to be the next Wizard King.
In my opinion, Asta is a great leader, even without holding an official leadership position. He protects and cares about everyone, and he motivates people without even realizing it (even the captains feel inspired by him). However, as I mentioned before, he’s still just a knight, and we can’t ignore the existence of the hierarchy.
Lastly, we know Julius likely had someone in mind as his successor. Even though he’s now a demon and the villain, he must have mentioned his choice to someone, probably Marx. And even if Julius chose Asta, I don’t think Asta would accept it right away. He’s not someone who would “skip” steps, and he knows there are people more powerful and more prepared than him. His dream is to become the Wizard King, but I believe he wouldn’t accept it if he felt there were others more qualified than him.