I suggest you follow on what you started with Easter, and incorporate more achievements and pvp within every event, it was nice to be pushed within a time deadline and to have goals set out with a decent balance between time/reward. It was also nice to be tempted away from:
halloween 2014: doing bk one after the other day after day
atleast with a mix of pvp/adventures and possibly scenarios? its varies the gameplay to break up the boredom.