Hard to pin down just one, as a lot of characters are interesting for either their design or subtle aspects of their character, yet so many never get more focus than being a plot device whilst others slowly fade from relevance the more the series went on.
As far as ones off the top of my head, I would've liked to have had Yamcha's backstory explored since this teenager having been brought into a life of robbing and murdering as a desert bandit makes an interesting journey. I'd have also liked to have seen the ideas of the Three-Eyed people that Tenshinhan has been stated to descend from in the Daizenshuu and
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interviews be explored more.
As for specific characters, Chaozu is one that quickly comes to mind. He's basically a living doll who's attitude in his battle with Kuririn made him pretty entertaining (emotionlessly charging forward in attack like a machine) yet he quickly got shafted into being Ten's cheerleader.
#8 is another notable one since his inclusion in the Cell Arc could've really enhanced the emotional beats. For example, let's remove the #16 we got and instead rewrite so that #8 is found by Gero and upgraded into #16. There's no need to change their peaceful nature when it's a trait they both share, but the programming forcing #16 to want to kill Goku now becomes a tragic one when this is against his own will. You could then have #16 spend the days of peace with the Son family after being fixed to highlight the friendship between he and Goku and make the impact his death had on Gohan far greater. The Path to Power kind of did this with #8's death and Goku's rage boost playing out in a similar fashion to Gohan going SS2.
Super Saiyan said:
For me, I’d have liked to see Roshi remain in a prominent role for more of the series. I really enjoyed his moments in part 1 and he really got the shaft once power levels started rising exponentially. Now I’m not saying to give him MUI like Fagotaro did, but keep him somewhat relevant in the story somehow..
That would go against his arc of judging his time to have passed and leaving things to the next generation. Of course, there are ways to go against that whilst still being consistent character growth. Whilst the Super Anime handled it poorly due to how cheap it was for him to reach Base Galu tier from doing regular training in secret to get Manly GAINZ, the idea of him having been inspired to return to training from seeing just how unimaginably far above him Goku and Kuririn could get is a nice premise to keep his character relevant.