Not just seasonal or anniversaries or DLC drops. To get more players back to areas that are almost void of players, ZOS needs to have monthly, weekly or bi-weekly 2xXP or 2xMats events in different territories, Coldharbour or Craglorn.
Before One Tamriel, world bosses could be solo'd. Now it takes a group, but there's no one around. With the Orsinium event, it was great to have all the players back, doing dailies and bringing down those skull and bones bosses.
Rotating events more often is a great incentive to get the players back, create new toons and level up those toons, which could mean more crown purchases, etc.
Start thinking out of the box ZOS.
PSN NA/EU: DesertDweller99
PC NA: KaktusKing