even if they are in different leagues, but no more than one league tier apart. In this case both players will be subject to the level caps of the highest rated player. So that means that if a Gold I player is matched with a Diamond III player, both players will be able to use max level cards in the match.

I think most gold league players don't have diamond level decks, so it doesn't solve the problem. It can be helpful only at the beginning of the season.