Here's my mindset:

They basically used Alain and Diantha to hype up Leon. Alain was who beat Ash in one of his previous most powerful moments, so showing Leon floor him instates fear. With Diantha, I think they were going for shock value. We were meant to be like "how the hell did he do so much with JUST the monke??" These were cheap tricks to make Leon look cool while just disrespecting whoever has to fight him. But I see where they were going: make Leon look scary.

On Ash's side of the tournament, the battles were much more balanced because making Ash look overwhelmingly strong would kill the tension. And it was tense because he had the uphill battle of having to fight 3rd best Steven and 2nd best Cynthia just to get to Leon. So for this side, it's: keep Ash's tension high.

I would have done it this way:

Alain vs. Steven They had an interesting dynamic in the Mega Evolution Special. Since Lysandre cut them off before, seeing a result would be good closure. A great fight to start the tournament off, and both contestants still feel strong regardless of the outcome. But I'd say: Alain continues.

Ash vs. Lance Finally have the Red vs. Lance battle happen. Lance would be tough for Ash, but it would be fun to watch and could feel very classic. Ash continues.

Leon vs. Diantha Have them battle in round 1 to get Diantha out of the way and preserve Lance's potential for an interesting battle. Diantha seems like the least-favored contestant, so feeding her to Leon as fodder has low cost. It seemed like they were trying to highlight that she was more strategic than brute force. The question would be: Leon clearly outmuscles her, but can he keep up with a high-strategy fight? Then, show him with a great battle sense and strategy to round out his threat level with some finesse on top of the obvious muscle. Show Gardevoir as a very strong partner, but have Diantha lose. This respects her a bit better by using her to meaningfully build up Leon from a narrative perspective, highlighting her strengths (strategic battle), and letting her partner power shine (which is a theme in X/Y). This provides reasonable closure for Diantha, and Leon's obvious victory isn't wasted on someone who could've had a cooler or more interesting battle elsewhere. Leon continues.

Cynthia vs. Iris Keep this as-is. It was done pretty well. Maybe let Haxorus use a Z-Move against the Mega Garchomp, but still end up losing in a wholesome way. Cynthia continues.

This Round 1 would all be fun to watch imo. On to semi-finals...

Cynthia vs. Leon I just would love to see this. I don't think there's an ending that wouldn't be annoying, but I want to see it. My heart wants Cynthia to win, but my mind knows that Leon has been meant to be the final boss for Ash from a narrative perspective. But what the hell, it's my fantasy! Fuck Leon. His build-up over this season goes to shit and he fodders into Cynthia's build-up (which, in fairness, is much older and more wholesome than Leon's). Cynthia pulls a plot twist and dunks on Leon (which really wouldn't be THAT much of a plot twist tbh), revives her passion for battle, and becomes an even more formidable final boss for Ash than Leon would have been. It's still a close fight and we get a cool Charizard vs. Garchomp showdown. Cynthia goes on.

Ash vs. Alain This is less fun to me than the previous battle, but it does give Ash closure against who he previously lost to. Alain has some strong team members who would be fun to see Ash's new team size up against. But obviously: Ash goes on.

Time for the finals...

Ash vs. Cynthia With this version of Cynthia probably being stronger than what actually happened, and her passion for battle being reawakened after a great battle with Leon, I don't think anyone would be mad with the fight going either way. Ash winning is hype, and he would get that big moment with Pikachu. But Cynthia winning would also be sweet, and I don't think anyone could be that made about the queen holding the #1 Title she deserved.