Dailies and itemized XP (XP per KO, round won, objective done etc ) should be introduced IMO asap to retain players. Once players start acheiving higher aminal levels, where you need more XP to level, people will start tapering off due to the monotonous grind.
The only XP to be given is by simply completing a round, winning and weeklies (and maybe acheivments?). We need more ways to level up. Simply put. This is a casual game, not a RPG grind house.