Instead of listing individual rewards on each series pass, perhaps they could reward token items – eg. "Level 5 Prize Coupon" – that you can then redeem for rewards of that level from any season.

I think it would have to lead to more people playing overall.

Possible scenarios:

Someone who has been here from the beginning and wants all the prizes will still need to play every season.

Someone who doesn't want every prize will still have to participate in one series per prize they do want.

Someone who joins the game now and wants the old prizes will need to play additional series to catch up and earn the prizes they want in addition to the new ones coming out.

Someone with multiple characters can play with different characters each series, instead of potentially trying to cram levelling all their characters into one series where they like the rewards.