Pokémon Battling, it is the literal beneficial factor that makes any Pokémon game good. How strong is this Pokémon or how balanced are those Pokémon? There is a large following behind the main trope of Pokémon, the battles.
And when you start digging around Pokémon games, you'll find that battles is ramped up in the difficulty department, via the Post-Game. You have so many ways to battle, that it isn't even quantifiable. There is so much a developer can do to give players, an awesome battling experience in the after game.
Technically, the Champion was supposed to be the best thing, to come with battling than Chocolate mixed with Peanut Butter. It was so good to test and satisfying to finish. This was the end...
The Final Boss. Nothing is supposed to test your skills as a trainer more then this battle.
Except there was.
The Post-Game of Pokémon had so many new things that it was nuts. New places to explore, more Pokémon, trainer battles whose levels were higher than that of the Champion (which really made me wonder, exactly how they aren't the Champion); It was insanity.
How many video game series can you say had as large of a Post-Game as it did the main game without paying for obscene amounts of DLC? Not a lot is there.
But where did we start? Oh yeah, battling. So the main thing here is that you can partake in more battles this time around in the post-game. Sure, you can catch a legendary or three... maybe five.... If you're lucky... But that wasn't what defined Pokémon. The battles defined them.
Look in a dictionary and find a definition of Pokémon. Somewhere in the fine print should say "cartoon animals battle each other for supremacy of the entire universe". Don't fact check me... just trust me, I am being 100% legitimate.
So let's talk about the actual battles. Well, what the hell do you do in the Post-Game. For starters (hah, get it {XD}), let's discuss Rematches.
GSC introduced this concept of a phone. And trainers would call you at the most inappropriate times to tell you how they failed to catch a Pokémon (no one cares Joey, no one ever did). BUT what was interesting was how you were able to be called by all the Gym Leaders, and arrange a rematch.
This time, however, their levels had improved and AI was expounded on. They were a bit harder to beat. And it was fun. The feature was also available to normal trainers too, but who actually knew who Abigal was, when everybody looks the same?!
You also had Battle Facilities. These places are pretty cool, considering that all you do in there is battle. We have the PWT, the Battle Tents and Frontiers, White Treehollow/ Black Tower.
What was so interesting about them was the fact of how, well constructed was and the different feeling of each battling place. There was definitely a difference in how you approached PWT compared to other places where you could battle. PWT had a more urgent feeling since all the Gym Leaders were back and out for blood. While the White Treehollow let you explore through a maze at your own pace, battling as you go. Two different feelings. It was fun.
Replicating that in a hack, the hacker decides to something similar. Have some building dedicated solely on battle challenges. Like, battle all the Gym Leaders again, or beat all these guys in some sort of Inverse Battle. The premise was the same as any of them, they didn't require you to go there to beat the main game. A small distraction for some extra content, thus adding to the Replay Factor of the game.
And the long and short of it, just giving the option for a harder difficulty for trainers and the small distractions of some battle challenge is all that you need to fill that void in your Post-Game.