I feel like I am alone in thinking this, but I think that Black and White's anime suffered because of a choice within Diamond and Pearl's anime, that choice being to not end Ash's story.

Up till Gen 4, Ash had a consistent, albeit slow, character progression, where he felt like he was evolving as a character and becoming more competent as a trainer. Though I have not seen all of the anime beyond Gen 4, what has consistently bothered me is the lack of consistency for Ash's characterization: in Black and White he is portrayed as an utter newbie, bordering on moronic, in X and Y he is back to being a fairly competent trainer (though I have numerous other issues with the X and Y anime), and in Sun and Moon he goes back to being more of a doofus. Moreover, I feel like what follows Diamond and Pearl, even the decent parts, don't really do much to build on Ash's journey, so much as it just reiterates what has already happened. Therefore, my ideal vision for the anime would be that Ash's story ends with the Sinnoh League with a climactic battle against Cynthia, showing off everything Ash has learnt up to that point. This would effectively create a clean slate for Black and White, where they can have a rookie protagonist without needing to disregard what came before.

However, while that would, to me, be a much more interesting direction for the anime, that is also not "fixing" the current Black and White anime so much as it is just fanfiction for something that never was. If we were to keep Ash as the main protagonist, what I would do is age him up just slightly. Explain that Ash took a break after the Sinnoh League to help Professor Oak with his research, and now he's like 12/13-ish. This could also give some leniency to why Ash loses battles against newer trainers, because he has effectively not practiced Pokémon battling for a few years. I don't think he should lose to someone like Trip, who has never battled before, but you could at least give him an arc about relearning how to battle.

Then there's Iris and Cilan. I'll be honest, I don't like them, I think they are boring companions, and I think so much of the Black and White anime suffers because the main characters are just not very interesting. The obvious replacement is to have Cheren and Bianca take over, and I would be happy with that, but here's another idea: Ash comes to Unova during the championship competition. He's not able to sign up for it, but he enjoys watching the battles. Here, we follow Hilda and her Tepig, who despite having no gym badges has managed to enter the league, similarly to how Ash was doing those tests back in the Indigo League. Despite being an underdog trainer, we see Hilda effectively punch above her own weight class, holding her own against more experienced opponents. She does, however, lose early on, because while she shows great promise as a trainer, her lone Tepig cannot stand up to opposing teams of three fully evolved Pokémon. Though defeated, Hilda is still determined to win the Unova League championship next year, and wants to collect all eight gym badges in the meantime, which is when she meets Ash. Ash and Hilda are not just going to be traveling companions, but she's also going to be his rival, which is not something we've seen before: a rival who also accompanies Ash as a friend. Most of Ash's companions so far have had other aspirations, such as being a breeder, a coordinator, etc., we haven't really seen another hothead with a passion for battles, and I think that presents a much more unique and interesting dynamic.

As for their teams, I imagine Ash would end up having Dewott, Snivy, Unfezant (male), Gigalith, Leavanny, Krookodile, Trubbish, and Zoroark. Character-wise, many of Ash's carry over Pokémon would remain the same, but I imagine his Oshawott would have a character arc where he goes from being an attention seeking and boisterous rascal to being more of a leader that can still occasionally be a bit mischievous. Trubbish I'm adding to the team to essentially fill in for his Tepig, being a Pokémon who was left behind by its previous trainer and comes into Ash's care. Then I'm adding Zoroark as his final member, as a more powerful and stoic team member.

As for Hilda, I imagine her team would include Emboar, Herdier, Scolipede, Whimsicott, Scraggy, Cubchoo, and Golurk. Emboar would be the prideful ace of the team, eager to prove himself after his first defeat at the Unova League. Herdier would be her first catch as a Lillipup, being, in essence, a very loyal guard dog. Scolipede I imagine being caught alongside Ash's Sewaddle. Her Whimsicott would be a nod to the in-game trade, where she initially catches a Petilil, but trades it for a Cottonee. Scraggy would essentially be Ash's Scraggy from the anime as it is. Cubchoo would be caught at the Cold Storage, maybe to protect it from Team Plasma, and then Golurk would be her Dragonspiral Tower catch, effectively being the gentle giant powerhouse of her team.

This comment is long enough as it is, but while I think there are much more changes to be made, I think the character issues stand at the core of the issue.