There were loose ends, and someone had to take care of them.
Most Naruto/DBZ crossovers overpower the Naruto universe like crazy or drastically depower the DBZ universe. Here's what would happen if you didn't do that and used the most logical way a Saiyan would come to the Elemental Nations and the end result of it.
A common cliché is impaled by the icy spear of reason.
Uzumaki Naruto keeps his promises. No matter the cost.
Naruto is doomed to repeat a cycle of stupidity and debauchery over and over again. He's not amused. This is part of the reason why most people think Naruto crossovers are doomed to be stillborn. What would the canon Naruto react like to the actions of those cheap copies?
It was just a small gift. But to Naruto, it meant the world.
After conquering all of the suspiciously Arceus shaped known world, what is left for the protagonist of Pokemon Conquest to do? Well, it's a good thing Arceus stuck around to alleviate boredom...
After witnessing a horrible event, Gohan asks a stupid question.
Saiyans get stronger when they're beat up and then healed. Freeza's planets have healing pods. Vegeta has a brain. Magic happens.
Gohan remembers the first to sacrifice for him. A tribute to the forgotten.
Naruto makes a joke Sasuke finds genuinelly funny. Still doomed, though.
A vial of polyjuice, an annoyed teenaged girl, two friends having a row, a date to a ball, a dog and a tournament. One has to wonder how certain events would've gone if certain things that happened were remembered when stupid mistakes were originally made.
It has come to Dumbledore's attention that several cliques at school have claimed that they run Hogwarts. Therefore, he writes a song about how things are really like. Based on I Run NY by The Lonely Island feat. Billie Joe Armstrong.
Harry Potter takes a page out of Agent K's book on how to deal with creatures infinitely stronger than yourself. It's a little... messy.
Harry Potter's summers are not very enjoyable. Thus, Sirius decides that he'll make this one at least a little bit better. Ibiza's Veela enclave proves most cooperative in this endeavor. Summer in between year 3 and 4 in the Simplicity Series.
Because a complicated life can only get more complicated when a murderer is on the loose and after you, Harry continues to focus on problems one at a time and on solutions of the simple kind. Third year on the Simplicity Series.
Basilisks? In my school? Surely you jest! In which a clever wizard finds simple solutions to the problems of a complicated school year. Second Year in the Simplicity Series.
Harry has a complicated life. Which is why he looks for simple ways to make it easier. Because being intelligent is not about being a knowledge sponge. First year in the Simplicity series.
In which very simple solutions are found to a complicated tournament's tasks by a clever wizard. Fourth Year in the Simplicity series.