Create your own scenario

In this timeline Constantine I was never born, therefore Christianity never became the lead religion and the Gladiator fights never ended. The gladiator fights would go on forever in history and spread through almost every country and city. Every major city would have these arenas for it was an effective way to deal with criminals.
In this timeline most Asian civilizations would not agree with such brutal games. This would not go to the standards of causing a war but it would cause propaganda from Asia to Europe. Which could lead to ceased trading between the two.
Gladiator fights would hit a mass spike after the discovery of civilizations in South Africa in 1487. Europeans would trade lots of valuables for slaves whom could fight as gladiators. With the discovery of Africa many new animals would enter the arenas.
With Christopher Columbus sailing the seas, when he reach the New World it could turn into a whole different story. Europeans would like to see how Native Americans can fight as gladiators. Native Americans would naturally have poor combat skills against trained gladiators. The Native Americans would see this as slaughter among their people. They would see Europeans as devils with such brutal acts, its likely they would become afraid and turn to war with Europeans. Native Americans would trade captured slaves for guns and weapons.
Europeans would still gain knowledge of guns through Asia. Though they may not be as effective because they would be behind with technology such as gun powdered weapons.
The Europeans would not be as giving and peaceful as they were in our real timeline. Europeans left Europe to leave the Roman Catholic religious ways to start their own. In this timeline Europeans would be looking for land to own, resources to take, and gladiators for fighting.
As the brutal war merge within the two, there is no doubt Europeans would win this war. When news reach Europe about an war over the Americas, people may see this as murder the same way we do today. Revolutions would start in Europe. As Native Americans would fight, every battle they win they would capture more and more gladiators. As most gladiators are criminals and mistreated slaves, they would revolt with Native Americans. Now with guns and trained close combat fighters, Native Americans would have a proper army. The gladiators would convince other gladiators to revolt. As seen in history, gladiators would revolt. They would take ships and sail back to Europe where their numbers would increase and Europe would be forced into civilized times as seen today. Though Native Americans would still lose the war with Europe they still changed history and may have survived as a race.