I had an idea for a somewhat simple challenge, koff~
Not to difficult to remember, but it might be harder later on down the line. This challenge can be set for any main game in the series, but I'd suggest trying it out on X & Y. I will partake in this challenge as soon as I receive my copy of Pokemon X Version. (and figure out how to sign up for stuff)
The Challenge: Only three pokemon can be used to battle throughout the game. That's it. Simple, right?
You can use any combination of any* three Pokemon you wish, but you must use only three at all times. The exception to the 'any' rule is this: You cannot use pokemon that you've traded for. You can trade pokemon, but you will be restricted from using them in battle. This is to prevent trading lvl 100 bidoofs and magikarps to mop the floor with. (Also lvl 100 Mewtwos, Dragonites, Metapods and the like)
The exception to this rule is when you're out catching pokemon (obviously they'll add the three that you caught to your party) but you cannot use these newly captured pokemon to battle and catch other wild pokemon. In order to be able to use them, you'll have to travel back to the pokecenter and swap out your party members in order to comply with the rules.
You can buy any item and use any item in this challenge that is available to you, however, entering trainer battles with more than three pokemon in your party will result in a frowny face and a heap of your own shame. Oh, and the challenge will fail.
Eggs do not count as party members. So as long as they haven't hatched, you should be able to enter trainer battles with them, as you'll still black out as normal. I advise against this practice, however.
You can breed as many pokemon as you want, but I suggest that you hatch the eggs near where you received them to prevent yourself from being caught in a trainer battle with a pokemon hatched one step before.
If you have a pokemon you wish to level up using an experience share, it will have to be one of the three pokemon you're using in your party.
There is no saving before a pokemon trainer battle. If they beat you fair and square, pay them their cash and be on your way. The same goes for the elite four.
There is no limit to how many times you can re-battle a trainer. Since you're only using three pokemon, so they'll need to be strong.
Battle complexes can be entered (Colosseum, Train battles, competetions, etc.) but you must still use only three pokemon. If a challenge requires you to have four or more, you will have to participate in them later.
Once you have beat the elite four in the course of this challange, these restrictions go away and you're free to do as you please.
Well, that's it. There's my idea. If you can think of any holes, please let me know and I'll find a way to stuff them, koffi~