Maybe, it can be some sort of leveling up system, sort of like in the Pokemon TCG Leagues, after you challenge a leader, they give you a blue card with 5 "EXP bars" on it, and every time you win a battle a leader and win, you get 2 of them filled in, and every time you lose, you still get 1 for trying. ONce you get all of them filled in, you get a special badge and a green card with 10 "EXP Bars", and after you complete that one, you gain a badge and a red card with 15 "EXP Bars". Once you have finished the red card, you have to bring all 3 completed cards, and challenge the "Champion" and if you win, you get a super badge, while losing forces you to get a new red card.
Some extra stuff
[spoiler4z3]Each card has 5 "challanges" on the back, and completing each one rewards you with 2 EXP. And maybe, if you have completed all of the challenges for all of the cards, you would get a special badge. (Some example challenges: Don't use a Fighting move in a whole battle (Blue), Switch out 4 times in a battle (Green), and 1HKO a Leader's Pokemon (Red)
when you get to 5 EXP on the Green card, you gain a mini prize
When you get 5 EXP and 10 EXP on the Red card, you gain a mini prize
Completing the Blue card gains you a badge
Completing the Green card also gives you a badge
Completing the Red card [u4z3]DOES NOT[/u4z3] merit you a badge, you must first go and challenge the Champion to receive it
If you are, for example, working on your Red Card, but did not complete all of the challenges on your Blue cards, and you happen to complete one during a match, you can choose to leave the challenge uncompleted and receive EXP for your Red card, or complete the challenge, but you will not receive any EXP for your Red card on that match.
There might be a special Yellow card, dubbed the Rival card, with 20 spaces on it, and you can ask for one at any time (when you get one, you get a sticker with "PPL Rival!" on it. At any time if you see someone with a sticker (or even if they don't) you can challenge them, and if you beat them, you get 2 space filled on your Rival card, lose you get 1. After your Rival card is filled, you can take it to any Leader, and they will give you a special badge.
You may only challenge a Leader once per card, per day. So if you have already challenged a Leader once that day, and you are still on the same colored card, you will not be able to challenge them, but if it is in the same day, but you have gone up to a different colored card (or restarted the Red card after losing to the Champion) you can challenge them. Also, if you are still on the same colored card, but it is a different day, you can challenge a Leader again. Maybe, the cards will have some sort of a signature/date place on the back under the challenges.[/spoiler4z3]
I tried to incorporate the Rival system, Leader/Champion system, and Challenges system all into 1. Maybe I could incorporate the Team Rocket/Plasma system that alot of people liked, by making there be Dark Blue/ Light Blue, Dark Green/Light Green, and Light Red/Dark Red cards, and Plasma could use the light ones, while Rocket could use the Dark ones, and you would only be able to challenge Leaders of the other team, but I feel like that would over-complicate even more than it now.