Hi I'm here for another suggestion
Since I've seen so many people return back to play RO2 might as well give them bonus reward to continue their journey, in my experience on other games they give a special reward for some player that return back to play their game, those rewards are simple bonus but it really helps the player to advance their character.
these rewards mostly contain levelling stuff and gear stuff, also to avoid misuse of the event to farm rewards, only account that was active and inactive for the selected period of time that will get the rewards or simply send them a promotional email to claim their rewards.
regards
some players from RO2