I wouldn't go as far as making this an entirely different challenge, but maybe a dream theme, since that's what these sorts of challenges go into. :) I would make it harder though, since it does seem a little easy for some people. The Mage would be a little difficult to pull off, since post-third generation had the physical-special split, so I would change that around. I would change the rogue to any Pokemon which can learn a status move via level up, and has a high base speed, and be able to learn Thief. The priest sounds a little difficult as well, as there are limited Pokemon for that, which means you would have to use Miltank for anything before 5th generation. I'd change that to any Pokemon which can learn Recover, Softboiled, Roost etc via level up. Tank should be a Pokemon whose both defenses are higher than both it's attacks.
As for my idea, I wouldn't mind a challenge which would eliminate most Pokemon for use. My challenge would be a challenge where you can only train Pokemon based on gender differences. So the idea would be that you have to train Pokemon which have a higer chance of being either Male, or female. You don't have to use that specific gender, but just Pokemon with the higher chance of being that gender. For instance, if you were using a female class, you would have to use Pokemon like Jigglypuff and Vulpix, whereas if you were using Pokemon which were primarily male, you could use Pokemon like any starter, or a Combee.