I'm hopefully going to be running a campaign soon, and I would like some advice and suggestions of things I should think about before actually taking it to my potential players as a "here's what I got".

The premise is thus, as will be told to my players:

You (as a player character) wake up one morning with a symbol on your body and a prophecy from a god in your mind. This would be startling, exciting, incredibly groundbreaking - if the gods didn't do this every few decades, choosing a bunch of mortals seemingly randomly, giving them a prophecy to fulfill, as a sort of 'geocaching'.

Its been 52 years since the last event. In the absence of any sort of name given by the gods for this event, people began calling it Cosmic Battlegrounds, since that's more or less what it is. There are 21 Gods in total, and every time this Cosmic Battlegrounds occurs, the Gods form groups and alligences, pick a mortal as their 'champion' and give them a mission or a task to do. These champions seek out allies, others chosen by the Gods of their alligence, based off the symbol they've been marked with, and go about their tasks, while competing with other groups of God-chosen champions.

From the early days of the Cosmic Battlegrounds, a single rule have emerged, not stated by the Gods, but definitely enforced - those who kill off their teammates are given swift and vicious judgement. Other rules have come about from the leaders and rulers of the countries and kingdoms - all citizens must aid those involved to the best of their ability, as long as it doesn't cause themselves harm. All other world-wide events happening at the time will be put on hold - such as coloseum tournaments, pilgrimages, even wars go into a ceasefire until a group of champions emerges victorious.

No-one actually knows why the Gods do this. There's typically no rhyme or reason for these battles, whether it be for something specific, something vague, an item, a relic, or even an experience - the Gods tell no-one. However, there is always some kind of benefit to it all, as the winning champion group get a wish, a miracle, as well as the winning Gods' domains becoming stronger for the next few years (for example, should the champion of the God of Nature and Life win, then crops and harvest will be bountiful for the next few years). Because of this fact alone, the chosen champions are more than happy to compete for the win.

I've come up with my own pantheon of Gods for this as well, 21 in total, 7 Good, 7 Evil, 7 Neutral.

I'm planning on one of the races of my players be at war with the rest of the continent, to bring some drama and strife to the party, but they can't do anything about it until the Cosmic Battlegrounds is over.

So I have motives for participating and motives for working together, the two big things I believe should be addressed for campaigns.

For you guys on the Reddit, the reason the Gods are doing this is to spread their own dominion and power, locked in eternal bouts of conflict with the other Gods, like a giant free-for-all battle of Age of Empires or Civilization.

So my question is, what else do I need to think about before this becomes a fleshed out campaign?