Old System - extremely low rates for girl drop but the rates increase over number of battles basically you could get all new world girls before the next world starts.
New System - Visual tracking of progress with flat extremely low rates for 100% basically world girls will take 2 new recent world maps to get with event and legendaries are only possible if bring back catch up event that is after LD3 but we were given a new legend so it might never come or legends are premium grind girls now.