Dragon Ball characters are full of endless potential. Since the power ceiling of "God" was broken by Goku, the door is open for any number of characters like Goten and Trunks to continue going further and further than they ever have.
Then again, some have either reached their peak or don't have that same limitless possibility due to either a lack of training or screentime. Master Roshi is among those, but several others don't hit the same peak that the Saiyans would.
5 Dragon Ball Characters who are maxed out
1) Master Roshi
Master Roshi is a longstanding Dragon Ball side-character, present since the first series. Throughout the series, his role has diminished from Goku's master to a retired and perverted minor character in the grand scheme of things.
That's not to say Roshi is useless or worthless, he still has plenty of wisdom to share and gave a great showing at the Tournament of Power. However, he proved and admitted that his fighting days were long over when he temporarily died and was forced to retire after helping to thin the herd of contestants with the Evil Containment Wave technique and his most powerful Kamehameha.
He also seemed to tap into the lowest form of Ultra Instinct, but that did a number on his already strained body.
He helped during Moro's invasion of the Earth in the Dragon Ball Super manga. He got distracted by three attractive female convicts, typical of him, and nearly got overwhelmed when they fused into one.
He's content to live out his days in Kame House with his turtle, island, and seemingly endless supply of adult magazines.
No disrespect was meant to Yamcha at all. He's more than proven a willingness, maybe too much, to help out. Unlike Roshi, he has certainly kept up with his training and has proven to at least help out with the various threats that have come for the Earth.
His only problem is that he's normally outmatched by nearly everything. He died to a Saibaman in the beginning of Z, nearly died to Android 20 in the Android Saga, and has not proven all that durable.
It was fun to see him win the Universal baseball game, though. He did manage to beat up some of Moro's crew in the Galactic Prisoner arc, so points for trying to help and managing to do more than Krillin or Roshi.
3) Majin Buu
Majin Buu's best moments were when he was a villain in the Buu saga. At least there he was, capable of beating and killing people, like surviving Vegeta's final explosion or Kid Buu being revealed to have slaughtered the Supreme Kais.
Following an attempt to hit Beerus that failed, being used by Mr. Satan for marketing purposes, and falling asleep quite a lot. But then again, he made a great showing in the Zeno expo against Basil. Then there's the Grand Supreme Kai awakened within him during the Moro arc to fight against the planetary parasite.
Other than that, he's maxed out.
Piccolo having a new form aside, his peak was going Super Namekian in Dragon Ball Z. He did this, fought Cell hard, and has since been more a mentor/backup for the Z fighters. Then again, that's not a bad thing at all.
Piccolo has proven far more durable than many give him credit for, able to withstand physical blows from opponents like Frieza, went up against Cell, fought well alongside Gohan in the Tournament of Power and then aided in the Moro arc.
Bottom line, Piccolo may have some potential yet to be tapped, but he's tapped out for now. The new movie may disprove this, and Super shows he's not out of it yet.
Krillin has died way too many times to count. Despite these deaths, he gets credit for coming back and trying again. He managed to survive Cell and the Android saga and even married 18 and had Marron with her. That counts for a lot.
For this list, however, while he's gotten much stronger than when Frieza killed him, he's not much when compared to the Saiyans or half Saiyans.
He still stopped the show in the Tournament of Power with his teamwork with 18 and the others before being the first eliminated. The Moro arc had him kill one of Moro's bandits. He was also able to survive long enough to get senzu beans to everyone.
At least, Krillin is one of the smarter Z Fighters trying to survive long enough to ensure everyone else does.
5 Dragon Ball characters with endless possibilities
1) Android 17 and 18
One of the duos on this list, the twin androids were already plenty strong by the time of the Android Saga in Dragon Ball Z. While they may have bit off more than they could chew with Cell, they were strong enough to beat down the Z fighters and would've most certainly slain Goku had they gotten to him.
By the time of Dragon Ball Super, 17 and 18 have more than gained enough strength and skill to continue moving on alongside the other main characters. They were able to last in the Tournament of Power, with 18 eliminating the rather powerful Ribrianne and 17 winning the Tournament by aiding Goku and Frieza in taking down Jiren.
17 also confirmed that the Androids, while they can suffer injuries, don't get tired as humans do. They certainly proved durable enough to last as long as they did.
Even though they're both involved with their families, they still lend a hand from time to time and prove that they can continue going if they so choose.
2) Goten and Trunks
It's a controversial choice, to be sure, but Goten and Trunks proved they could fight during the Majin Buu saga with their Super Saiyan abilities and the fusion of Gotenks.
The two helped keep Goku and Vegeta alive during the Copy Vegeta saga alongside duking it out with the main villain of that saga, helped fight Frieza's forces, aided Future Trunks with the time machine while helping him get his confidence back, plus kept Android 17s island safe and fought off the angry Cell Juniors.
The fact that they are confirmed as showing up in the upcoming Dragon Ball Super Super Hero film says they have some relevance to the proceedings. Plus, and this cannot be stressed enough, they're barely children and are still growing, so they have a huge amount of potential to be tapped.
A newcomer to the Dragon Ball Super manga, Granolah is the last survivor of a long-destroyed race called the Cerealians. The Saiyans destroyed the Cerealians on Frieza's orders, and the Heeters hunted down Granolah himself.
Bardock defeated the Heeter Gas to buy time for Granolah and Monaito to escape after the lead Heeter Elec killed Granolah's mom. However, Granolah still hated Saiyans since he was a baby.
Granolah has already proven his worth and weight by fighting both Goku and Vegeta and practically winning over them both. Gas then got involved and shattered his arms.
Although Granolah is down as of chapter 81, the Cerealian Warrior is still growing and has plenty of potential. Anyone that can survive Vegeta's Ultra Ego and Goku's Ultra Instinct deserves praise!
Goku's first son still has plenty of potential. The problem is he doesn't want to fight, which is fine. He has shown more combat capability and hidden depths during the Cell Saga and every saga afterward.
He stood against Super Buu, was great in the Zeno Expo, was able to fight off Frieza's forces and even withstand Frieza's blows, not to mention his great showing in the Tournament of Power.
Despite having some rust on him, Gohan can still go and still has the potential to be a better fighter than Goku, or at least at his level. He's no Super Saiyan God, but since when does one have to be to help defend the Earth? Yamcha manages just fine despite not being a Saiyan!
He's also set to be a main character in the Dragon Ball Super: Super Heroes movie, so he gets very strong.
An obvious choice for potential and endless possibilities, Broly is Dragon Ball's berserker character. In the Dragon Ball Super: Broly movie, the berserker could manhandle Vegeta, then go up against Goku. That was before he started berserking. When he did berserk, he needed the fusion of Vegito to stop.
So, that says he still has potential. He and Goku may duel one day. That would certainly be something to witness for people who were fans of the old Broly movie. It's also cathartic to see Broly beat up Frieza and generally see how his character develops when the Dragon Ball Super: Super Hero movie releases.