Good question! I have a few candidates I want to mention:
Indigo League (Kanto/Johto): Well... I'll admit I still think Koga would be an interesting Champion, because he is supposed to represent the more "traditional" and "authentic" aspects of the region, and his emphasis on gimmicks rather than his signature type would make for a fun fight. Of course, that depends on how you define "traditional" and "authentic" - because, in my point of view, he represents an idealized-to-the-point-of-bastardization version of the tradition that engulfs both regions, and that alone, along with the fact he takes his vision completely seriously, makes him (and his daughter Janine) inauthentic. That said, he'd be ideal as a gray or outright villainous Champion - someone who is corrupted by his own narrow-minded vision of the world over time and usurps the position from whoever is holding it at the time.
Bonus Indigo League: I just remembered that going by the events of Ultra Sun/Ultra Moon, Gladion spent at least a month away training in Kanto and came back with a much more powerful team to challenge the player character as Champion of Alola. I'm not gonna lie, the thought of him having become the new Champion of the Indigo League has crossed my mind more than once. Given that the ending of the Ultra games gives both him and Lillie much fairer fates than the original Sun/Moon, I actually want to see that happen now. As sad as it would make me to see him separated from his sister and from the player character, I don't want the guy cooped up managing Aether Foundation; I want to see him out and about and eventually becoming a world-famous Trainer!
Sinnoh: Playing Let's Go Pikachu and seeing the similarities between Barry and Trace makes me think that having us fight Barry as Champion would have been much more interesting than fighting Cynthia. Of course, I acknowledge that Cynthia is one of the most powerful Champions in the series, but... I never really vibed with her, and even less so after learning about her lineage in PLA. Barry, on the other hand, is one of my favourite rivals on account of his having actual character development, and I believe that him getting serious and making his way to the position of Champion would have been a perfect conclusion. Heck, I would love it if a future game showed us a grown-up version of Barry as the Sinnoh Champion - a powerful Trainer, but still the same dork he always was, and much more like his father Palmer than we were allowed to see in the Sinnoh games.
Kalos: As much as I like Diantha as a character (a Champion who does her own thing and doesn't follow you around for like half the game, yay!), it was rather obvious that she was looking to retire from her position so she could devote herself fully to acting. If a third version/reimagining/sequel of the Kalos games had been a thing, I would have liked either Korrina or Trevor to be the Champion. Korrina applies more to the idea of a sequel; she already has the connection to Mega Evolution as well as the competitive-but-still-friendly attitude, and only needs to adjust her team a bit so that it isn't beaten so easily. Trevor applies more to the third version idea; I know that his goal aligns more with being a Professor, but it would have been interesting if his applying his knowledge to Pokemon battling eventually led to him being the last opponent the player character faces. We've never had a genuinely nerdy Champion (sorry Steven), and Trevor would have perfect in that role.
I'll also answer the question in reverse and say that, after seeing him in both the Johto games and LGPE, Lance would be much better suited to the position of Blackthorn Gym Leader than Champion. When you meet him in G/S/C, he doesn't even mention his title, and in HG/SS he even helps you fight Team Rocket before you even find out he's the standing Champion! It's clear he just likes to help for the sake of it regardless of what position he's in, and I believe he'd be much happier if he could do so without having to shoulder such an important position as that of Champion. Of course, there's also the possibility he left home to become such a powerful Trainer simply because he couldn't stand being around his a**hat cousin, but, you know... it's only a matter of time before she gets herself removed from the position of Gym Leader for her bad manners and lava pools, and the Gym's gonna need a replacement. That's the Tohjonian way - the Champion ends up running a Gym, and who would be better for that task than Lance?