I think the moral of the story here is the more cards in volume released the cheaper they will be. As saints forever has stated you only have to look at dynamic to see there is no value to anyone in releasing to many cards to a set. This is why ESP cards do not hold there value. Goerge,s idea is right,1 parallel set to each release in all the same jersey every parallel set a different jersey.
If they are intent on making 2 parallel sets then I agree that there needs to be something different about them other than just the colour. On the rarer parallel set why not remove the player details on the back and replace that with an image of a part of the club logo. Like a jigsaw parallel, and when all 9 cards are together you get the full club logo and club stats (date joined, premierships etc). Would look good especially while the team sets are 9 cards.