It would make sense if the fact that Ghetsis wasn't the "champion" meant that considering the game "complete" once he's defeated made challenging Black or White easier than the others, but it doesn't. For all intents and purposes, they're pretty much exactly the same at that point and if anything, Black and White are already more challenging, since you have to defeat TWO high level trainers after the E4 instead of only one.
Rather than fixating on the word "champion," the goal should be to make the conditions for "completing" the game essentially the same for all the games, so that there's no situation in which players are essentially rewarded for playing one game and punished for playing another. To that end, it's self-evident that the conditions for "completing" Black or White should be the same as the conditions for "completing" Black 2 or White 2 - defeating the E4 and <whoever comes after them>. The fact that <whoever comes after them> possesses the title of "champion" in Black 2 and White 2 and doesn't in Black and White is ultimately a meaningless distinction. If that fact was ignored, then the requirements for "completing" a challenge in any of the four games would be essentially the same. Since it's not ignored, the requirements for completing a challenge in Black or White are much GREATER than they are in Black 2 or White 2, and that simply makes no sense at all.
Again - I can appreciate that you're loathe to change a longstanding rule, and I certainly don't blame you for that and I mean no offense by any of this. It's just that, either way, I still think that it's a poorly considered and counter-productive rule. If nothing else, hopefully these posts will encourage future challenge creators to not include it in their own challenges.
And all that said - thanks, sincerely, for tending to this challenge thread, even in the face of people complaining about the rules... :D