I'm not even sure what the theme for the season. that may be my own fault and im not paying attention enough but imo a season's theme should just jump at you. but i have a suggestion for themes and how it could play into the games cosmetics as well.
the main part of my suggestion is that you guys do like a soccer around the world kind of thing.
i've got like 3 example off the top of my head.
I heard like a brazilian beat playing in the background the other day and brazil is definitlely one of soccer's biggest hubs. in a brazilian season you could release some brazilian streetwear like the Havaianas everybody wears, maybe some brazilian dancing emotes, and some players like neymar, Vinícius Jr, and pele. you could add like a stadium in the center of the favelas as well.
you could do a japan themed season. they also have iconic streetwear. you could secure a crossover event with blue lock, captain tsubasa, or aoashi, any of them would give you plenty of emotes/ goal celebrations to work with.
the last one I can think of of the top of my head would be a season based in england. david beckham or harry kane, roadman fits, UK drill. Also you could add like the chainlink fence stadiums they have in the UK.