The Skirmisher league was renamed to Fighter league to avoid confusion with the singleplayer mode, which is closer to the Builder league than what it is intended to be. The wording is still bland but I find it suggests a lot more that there will be a lot of action and aggressive plays.
The leagues are currently only informal, as nothing make them apart other than their players. With more maps under construction, there will be changes in the next version. The map pool will be different between leagues, with the Builder one focusing on large maps with at least medium oil and more straightforward ones, while the Fighter league will use maps from all sizes and oil levels to encourage diverse strategies.
For example, High Ground Pass, the low oil small map, is already removed from the Builder league with this change. Vision, the large medium oil diagonal map will be replaced in the Fighter league for new one with more use of the sides.