I really like your idea , but it might be hard to be realized .
What about people who use several decks , what color do they belong to?
Same for multisphering.
The teams should be guilds , or maybe random teams changing each week or something.
Winners should get like 10 or 5 gran , or a random 1-star card of the winning color or faction .
Like : Begin of the week : You are assigned to Mercenary Nation team .
Your team wins !
You get Heirrat Mage ! Or get to choose between few 1-stars of the Mercanary Nation
Maybe , if you did the most you can get a 3-star maybe ?
(Maybe also add Factionless 'Insert Color Here' nation or something.)
(Nation with alot of high rarity cards (Abyssal Forces) Should get only a few members , but if they win they get a high rarity)
All just ideas aswell