Many Pokemon characters (and pokemon themselves) could use some tweaking here and there, but none as much as Leon. 

Absolute disaster of a design. Its a slap on the face. If they were going for a "regal" look, they could've doubled down on the concept and that alone would improve it a lot. And who knows, maybe give him an obnoxious and more antagonistic personality/role to go along with it. 

On second tought, the original design might just be more fitting for the trainwreck that Sword/Shield are. 

Also, keeping incineroar on all fours, without changing that much else, would've done wonders for it. 

Bob-ombs need a redesign.

I can't think of any.... But I do wish that Sonic Boom Knuckles' design was transferred over to the mainline series. I know I am in the minority, but I loved it.

The main characters from Final Fantaxy X-2. Didn't fit at all visually, and I seriously wonder how Yuna regressed from a fairly mature person to an immature teenager.

That Daisy design is pretty great. The Amy Rose design is cool, too.

I've never liked the hedgehogs in Sonic looking like they have one massive eyeball. I wish they would separate the eyes, even if it would be controversial.

I'm currently playing KH series from start to finish. I don't know if Nomura has some kind of thing for long black trench coals or something but every main enemy is in them. It's actually silly and legit takes me out of the threat of them as they are laughable (of course their dialogue is something to be desired too). It's like they literally have a uniform and they have to wear it, even some characters who aren't bad guys stay in it and it's odd. Not even practical for fighting either.

The cast of Xenoblade 2.

Make Sonic look like Sonic 2 again.

Japanese Sonic (early games) is peak Sonic. 

Street Fighter V Ken

Wasn't a huge fan of his new default look in SFV. His half dress Gi and body suit and hair looks kinda bad. Hopefully they'll make him look more his usual self in VI.

Lara Croft - Rise/Shadow of the Tomb Raider

From iconic green tank top/glasses to generic grey action hero(ine) attire. For the next inevitable reboot I'd want something to make Lara stand out with a more iconic look and personality.