Pet Charms are fine and all, as is any type of similar currency, but it doesn't necessarily add anything to Pet Battles. It's just a grind to collect as many as you can to spend at the vendors. There's also stagnation - the only thing that really changes from expansion to expansion is where the Pet Charms come from.
So what if there was a new form of currency? One that adds gameplay value and transitions well into each expansion?
Trainer Fame. The currency you never knew existed (because it doesn't) but you have to have. Something to consider for a future, future expansion.
What is Trainer Fame?
Think of it as something akin to PVP Honor points. It's not an item that goes into your bags; you simply have it on your character and NPCs recognize this. It's "fame" after all, right?
How would you earn it?
Here's the fun part! Fame would not be a static (or random) amount. How much you earn depends on how well you do in a PVE Trainer battle. The quicker you defeat the opponent, the more Fame you earn.
If you defeat a Trainer in 10 rounds, you'll earn 5 Fame. Defeat him/her in 5 rounds and you'll get a whopping 10 Fame. Just as an example. Values can be tweaked, of course.
The goal is to defeat Trainers in as few rounds as possible. Show off your prowess as a Pet Battler, earn notoriety, and then use it to buy awesome pet stuff!
Not only would it give us currency to strive for, it adds a challenge to each PVE encounter. And it continues to add that challenge to newer and even older content. Pet theorycrafters and battlers would constantly be refining teams to get wins with the fewest possible rounds.
Even once a "perfect" strategy has been created and readily accepted by the community, it wouldn't be the go-to strat forever. New pets are always being added, which can greatly impact strats and teams. 'X new pet cuts down my rounds by 3. I can earn a few more Fame points!'
Other mechanics could be added to this currency system as well. Just off the top of my head... ranks? Um, sure. Earn enough Fame and get promoted to the next Pet Trainer Rank. Perhaps certain items will require certain ranks: 'This item requires 50 Fame and Pet Trainer Rank 2' or something like that.
Fun interactions with pet-related NPCs depending on your Fame and/or Rank could also be interesting.
The downsides to the idea of Trainer Fame...
I could see newer/casual battlers feeling left out, as they may not earn a lot of Fame in the beginning due to their smaller pet roster or lack of Pet Battles insight. But more Fame would come as they progress and gradually learn the mechanics. There's room for growth.
There was an argument that once a strategy was created, it would spread around like a virus and nothing would change. Everyone would simply look up teams to use and any type of possible challenge would cease to exist. While that's true, it's also true now. Strats are easy to look up.
The difference is that there's no incentive to change anything or try to put together something better right now. Currently, if the community creates an acceptable team composition for a specific fight, that's it. It's done. Everyone can look it up and use it. We move onto the next one.
But with Fame, there's incentive to KEEP refining strategies. We all want to earn more, and with a dynamic currency like Fame there's a real possibility of being able to. So some will continue testing new ideas and trying out different teams, all to get the best possible outcome for the highest reward yield.
Plus the ever-changing battlefield of pets would give players a reason to continue experimenting with teams and strategies. Fame would give us a reason to keep going, and new pets would give us the means to do so.
Would it make content feel mandatory since some players will want to min/max their earnings? If there's the potential to earn/collect a reward, players will want to do it right? That's always a big concern, and if yes, perhaps Fame could be limited to only current content. Something along the lines of, 'Defeating older Trainers is so yesterday's news. But I hear there are some new Trainers that are looking to prove themselves...'
How would we transition from Pet Charms to Trainer Fame? One way would be to redeem them at a pet vendor. Or if there's an internal tracker that keeps track of how many charms your account has earned, perhaps the appropriate amount of Fame could simply be awarded to you at the start.
Right now I just don't see that many downsides to this idea. The pros seem to outweigh the cons. Am I missing anything? It's possible that I'm just too biased.
Anyway, this is merely one idea out many great ideas for Pet Battles. Something to catalog for consideration in the future. :)