If it's a pokemon used in a common or even rouge tournament viable deck, it's banned. We still allow the use of any trainer or supporter or item card so the weaker decks still function well. And depending on what the people want, also pokemon may still be allowed that don't necessarily attack but help with the deck engine like shaymin EX so the lower tier decks have a better engine to work with. This mode gives a bunch of cards the ability to thrive and be played when they would otherwise not be good enough to be played and lets us see completely new decks to play with built around them. Without the better cards to keep them in check how well will they do? What will the format look like? And maybe after further consideration some specific trainer card bans might be introduced like super energy removal and energy removal in base set so the pokemon with higher energy cost can be used.