I suggest that since we need new players, or more active players in each universe...You make a reward system run weekly in each universe... However it switches from universe to universe, in example.

Speed universe Week 1: Special event occurs rewarding players with rubies, resource boosts or even resources themselves.
Xtreme universe Week 2: A different event occurs rewarding players...
Massacre universe Week 3: A new event occurs providing the players with a reward at the end of the week...
Standard universe Week 4: Once again, another event comes around.

This will keep players cycling through universes, encourage players to try harder in every universe, create activity and maybe even draw single universe players into a secondary or third universe to play... This will make a balance of players in each universe theoretically and potentially allow the smaller players catch up to the big dogs enhancing war, adaptive tendencies and so on...