The old heroes have all gone now. Passed on into legend long ago, with none of their calibre to be seen again. No one knows how, or when, or why the ki began to fade. Most don't even realize the world has changed, they just go about living their lives the way they always have.
The metropolis has spread across the Earth. There are no longer cities, just sectors of the great urban sprawl. The natural places have largely been paved away decades ago, with only a few nature preserves kept for the few wealthy who can afford to maintain them. When they ran out of space, they started building up, layers upon layers, until the highest points literally scrape the sky.
The trees have all gone. The lakes and rivers and streams. There are no farms because there is no land left. All the food is engineered, buildings are crafted from synthetic polymers. Even life itself is no longer immune to the reach of the chemist, as the factories are more and more of them are manned by the custom-manufactured Universal Humanoids of the Shinigami Corporation.
The Shinigami Corporation is the most powerful organization on the Earth. It has essentially become the government, the economy, the police, and the military. One rule, under the mighty power of the corporation. One nation, under the CEO. And it was the Universal Humanoids that made them their wealth.
The Universal Humanoids, or Unimen, are custom lifeforms, down to the molecular level. A Uniman can be engineered to live off of almost any substance, to work without tiring, or sleeping. Even his level of intelligence can be designed, anywhere from simple animal to better than human, though most are kept more towards the former.
The present presiding officer over the Shinigami Corporation, is Dr. Shinigami himself, the man who founded the corporation, and invented the Unimen. Little is known about Dr. Shinigami, understandable given the immense size of the organization. With the exception of quarterly addresses to the masses over the videonets, he is never seen by the outside world.
Forgotten, But Not Yet Gone
But where do the Dragonballs fit into all this? While time has largely forgotten them, they still exist, though they have changed. The slow draining of Earth's Ki has left them without the power to manifest physically; instead they take on a more ethereal form.
But another even more important change has befallen them. As the Earth's Ki waned, it awakened a kind of mystical self defense mechanism. They have an agenda now, and that agenda is to put a stop to the force that is draining the life out of the Earth.
To do this, each Dragonball has sought out one individual, and merged with them. In so doing, that individual is imbued with some of the lost power of the great heroes. He also receives guidance from the Dragonballs, who speak to him always in the back of his mind, urging him on towards their ultimate goal.
The Hunt is On
But the battle will not be easy. Shinigami has learned of the Dragonballs, and coporate forces scour the great sprawl for their newly awakened carriers. The Corporation fears them, as potential champions for resistant forces in the underground, but also desires the Dragonballs, so that it might abuse them for it's own purposes.
The soldiers will, if possible, try to capture the heroes, but if that appears impossible they will not hesitate to use lethal forces. While Dr. Shinigami desires the Dragonballs, he fears the power of the new warriors more, and the threat they may bring to the Corporation.
Thankfully, our heroes are not alone. The Celestial Palace still stands, but to protect it from the forces of evil, it is now seperate, yet a part of our world. It exists as a pocket dimension, that can be accessed where ever there is still growing life. Standing trees, a potted plant, any of these can be used by the possessors of the Dragonballs to open a portal into the Celestial Palace.
Here they may find solace from the harsh city, and those who would hunt them to oblivion. It is also a place to train, and to learn. And it is here that Dende still watches over the Earth. He is aged now, and weary, but he still has much to teach the young warriors, and his healing powers will prove a welcome aid to the warrior fleeing from battle.
The Dragonballs seem to select heroes almost at random, so almost any character concept will work here. A hero could be an office clerk, a homeless dreg from the lowest depths of the city, one of the resistence fighters, one of the Unimen, maybe even an executive higher up in the Corporation.
With the Ki on such a massive wane, you won't see the kinds of massive power levels you saw in Z or GT. More likely you will see something more on the level of the original Dragonball, essentially high powered kung fu, with only the occasional smaller energy blast. Still it's possible that characters could eventually learn something akin to the Spirit Bomb technique, allowing them to draw power from a much wider area. This of course would come much later in a campaign, as it would no doub require considerable training to master.
The Dragonball Z roleplaying game would of course be the logical recommendation, and would actually work better here for Legends than it would for DBZ, due to the relatively low power level of starting heroes.
For an even lighter, cinematic sort of campaign, a modification of Steve Darlington's There is No Spoon Matrix roleplaying game might actually work rather well.
Exalted may do well for a more crunchy sort of setting, as it seems to have considerable detail when it comes to martial arts.