(This is a What-If story that takes place right after the defeat of Cell, it's currently a work in progress enjoy)
Cell had just been obliterated by Gohans Kamehameha, the whole world was in celebration that the Earth had been saved. Days after that the Earth seemed to have returned to normal, the Z-fighters lived in peace... All but one. Yamcha, Yamcha was angered that he had little use to the team, he had small intelligence, and the only girl he had ever made a connection with was now with Vegeta. He felt immense rage he wanted to become powerful again and most of all destroy Vegeta. But Yamcha knew there was nothing he could do about this, after all he was limited by his own human limitations, no matter how much training he could do he could never match up to a saiyan, even his training in other world wouldn't be able to get him on the same level. He had severe depression and it only grew with time not even Puar could help.
Simultaneously the armed forces are feeling helpless, as their full force attack on Cell did absolutely nothing to harm him. They began investigating further on just who was Dr. Gero and more info on the Red Ribbon Army. They were able to gain additional data through a lady bug bot. It included DNA specimen of Gohan and Vegeta they studied it and learned more about the structure of saiyans and clues to their powers. From the info on the bot they learned of Geros secret base, however when they arrived the entire base was completely destroyed. Things were looking dim, they knew about powerful androids which they couldn't replicate and saiyans which appeared to be genetically superior to humans in everyway. They began taking applicants for a super human upgrade anyone was eligible, however, everyone they tried to inject saiyan DNA with was unable to survive the change, their bodies weren't physically powerful enough. That all changed when a devastated Yamcha showed up at the door and ready to enlist.
The recruitment officers did not think much of Yamcha because he did not look very powerful but they still allowed him to apply. Before the actual DNA injection was done the host would have to prove they would be worthy in a series of physical and mental tests. Yamcha even while holding back immensely outscored all the other applicants he was a favorite among the leaders. The day finally came where the surgery was going to take place. The general appeared at the event as well to oversee operations. Due to Yamchas great physical stature, he thought it would be possible to try a dual operation where scraps from Android 16 and Frieza would be fused with him to not only add saiyan powers but also give him the durability, and power of an android. The hungry power Yamcha welcomed the opportunity. This would allow him to conceal lots of his power as it would be in mechanic form. The operation soon began, Yamcha was injected at all sides simultaneously with Saiyan DNA it was more painful than he could imagine strange thoughts and feelings began rushing through his body the moon, a tail, and oozaru. He didn't know what to think of it all and before he could, he was put to sleep for phase two of the operation. When Yamcha had awoken from his brief slumber, his vision had changed, he could now see information of every single person he was looking at, including power level. The surgical team looked in awe of their work. They held Yamcha within the facility to see how he was performing and yet again he outperformed everything, even handling 500x gravity with ease! They eventually released him and said they would call him back when another Cell like threat occurred. Yamcha knew well enough not to rush into battle like his last incidents so he began training like he never did before he began traveling to distant planet's training and gaining power. Due to his android parts he was able to breathe in the harshest conditions. It was here he truly pushed himself to the limit (partially due to his saiyan genes) he went to planet Yardrat to learn the instant transmission, due to his extensive memory and power he was able to learn it within a week. He then teleported to King Kai even with his planet torn, apart he wanted the Kai to train him. At this time Goku was doing further training in other world so he did not see Yamcha. Because Yamcha was a former student of King Kai he gladly taught Yamcha the Kaio-ken after seeing now that his body was able to handle it. After this he bid his master a final farewell. Yamcha then does an instant transmission to appearing right next to Bulma. He then states "Tell Vegeta to come out, we have things to settle"
Vegeta came out of the Brief house with a wide smile on his face. "Who let the weakling come here?" Bulma told Vegeta to settle down. Yamcha shouted "Your reign of tyranny is over! Once and for all Vegeta, I came here to destroy you." Vegeta boasted a great laugh "I can destroy you using just half of my base power." Yamcha announced "We will just see about that!" Yamcha ran a powerful charge towards Vegeta he ran a swift punch in Vegetas gut the strength of his punch knocked the wind right out of Vegeta. Yamcha then used a roundhouse kick right across his face. Vegeta was sent straight through a nearby building. "How dare you! How dare a weakling human like you attack the Prince of all Saiyans!" Vegeta was incredibly angry. How could he gain so much power? "This ends now" he shouted. Vegeta fires a full power Galick Gun and Yamcha effortlessly blocks the attack. Vegeta then turns Super Saiyan and makes a rapid rush attack. The two go at it in hand to hand combat it appears that there is no clear winner. They are fighting at such a high pace that not even Bulma can tell what's going on. Yamcha then shouts "Kaio-Ken times two! " Yamcha then uses a swift kick to Vegetas side, knocking Vegeta into the ground. Vegeta was in great pain "How could that Earthling gain so much power? He wasn't even a much for Gero less than two years ago!" Vegeta entered a rage he began blasting as many blasts as he could not caring about who he hurt. He was too angry to care. Yamcha announced "It's time to finish this" he comes down and knocks a solid hit right into his head knocking the Prince out. Lifts Vegeta up flying in the air, he reaches the Earth's atmosphere let's Vegeta float for a brief moment, then forms a cup with his hands, he begins shouting "KA-ME-HAME-HA! Times ten!" A powerful blue blast came from Yamchas hands and Vegeta was blasted off into deep space left for dead. He then came back down to the Brief house with horrified expressions from the family and it's neighbors. Yamcha had changed with the Saiyan blood and massive power boost he could no longer control his own anger. Someone had to stop him.
King Kai could feel the loss of Vegeta from Earth, he knew he had to find someone to be able to fight the now corrupt Yamcha. Kami sent out telepathic messages to Piccolo, Gohan, Tien, Krillin, and Chiaotzu. He informed them how power Yamcha had begun, he doesn't know how powerful he really is but he does know Yamcha is part Saiyan now and that should be taken to account for as well. The government learned of Yamchas outbreak due to news casts, they were informed if he were to take out anymore super powered individuals it would have to be in a small populated area lowering human casualties. Yamcha agreed with the orders as they provided a large source for his power. Yamcha then went into the Wastelands to further train and meditate. After of few hours of trying to master all of his power Krillin, Tien, and Chiaotzu arrive on the scene. Tien said "Yamcha, how did you get so powerful?" He responded with "well you'll have go fight me to find out." A wide smile goes along Yamchas face from the thrill of fighting. "Don't make me do this" Tien warned. Yamcha put two fingers on his head. He had vanished. Suddenly Chiaotzu made a yell of pain, he was knocked out cold. Krillin flies up and powers up, Tiens rage begins to show he sees Yamcha going up right behind Krillin, Tien shouts "NEO-TRI-BEAM!" A large yellow blast slams into Yamcha he merely shakes off the attack. Yamcha uses several ki-blasts leaving Tien and Krillin both badly injured. Before Yamcha can use the final hit to kill off Tien. Piccolo and Gohan finally arrive. Piccolo shouts "release him Yamcha! What's gotten into you!" Yamcha replies with "I guess I'm just done being in last place." Gohan and Piccolo charge up and begin double teaming Yamcha moving at faster than light speeds. After Yamcha gets the best of them there is no sense in holding back for the pair. Gohan charges up all the way to Super Saiyan two, Piccolo powers himself as strong as he had ever been in his life. The battle looks stalemated Ki blasts were exchanged faster than the eye could see it looks like Gohan had finally gained an edge in terms of power then a shock came to the pair. Yamcha screamed "KAIO-KEN TIMES TEN" his power rises well beyond that of Super Perfect Cell. One punch by this buffed up Yamcha is enough to deliver a paralyzing punch to Piccolo. With Piccolo defeated Gohan becomes enraged his power rises several times. He throws a punch right into Yamchas face and for the first time Android Yamcha has been hurt. He then kicks Yamcha in the stomach, and throws him to the ground. This was the greatest attack anyone had done on the Android so far. Eventually Gohans rage begins to slow down. This is where Yamcha decides to launch his finishing blow "Wolf Fang Fist!" He shouts and knocks out Gohan. Cold. Yamcha had just defeated anyone who would be able to ever get in his way.