Dragon Ball
This is perhaps the most fleshed out one I've got so far as the folks who received my PM can tell you. The short version is this: the final generation of Z Fighters stopped the end of the world 300 years ago. Problem is it cost them their lives and the damage to the Earth was downright staggering. Billions dead, many if not all of the Earth's cites destroyed, the loss of 500 years of technological advances. The typical "let's wish everything back" plan is not an option due to no Dragon Radar and extremely poor knowledge of the Dragon Balls among the general population. Meanwhile the various skills and knowledge of the Z Fighters is all but gone due to their few surviving students being minimally trained at best. Ki usage is a lost art now and while Saiyan blood has spread quite a bit among the population they tend to be treated like second class citizens due to many holding the belief that their overwhelming power at the Earth's peak is one of the things to lead to it's downfall. In recent months a group of outlaws are beginning to display levels of power not seen since the time period prior to Goku's arrival. Somehow they seem to have uncovered some lost tech and rumors are floating around that they seek the Dragon Balls which by this time have fallen into myth. With no options left Earth's current guardian must now seek out a new generation of warriors to protect the Earth once more.
Dragon Ball is something several of us know quite well. Power level wise I'm actually looking at pre/early original DB levels for two reasons: it's rare that those levels are touched and it can hopefully quell any potential power level arguments. Meanwhile I've decided to go with a post apocalyptic Earth for a new twist. Usually DB is either about the race for the DB's or the prevention of Earth's destruction. The first element is there due to it being damn near a requirement for DB but it's not very often you see an "after the end" scenario that doesn't involve the Mirai/Future timeline.