I would like to suggest the ''Vampire Pass'' with Seasons.
3 to 6 Months per season, until a new one comes out.
The Vampire Pass consists of tiers. Could have 100 tiers in total. Each tier either has a reward or just an empty tier to reach higher rewards. The rewards could mainly be Bound-Items. These Vampire Pass rewards can be: Titles, an amount of Gold or Plat, Outfits, Pets, Weapon Skins possibly, basically anything. (Leave your suggestions below of what you think rewards could be). This can also be a very nice way to get Plat in the game. Let's say per season you can get 50-100 plat if you complete all tiers of the pass.
To level up through the Vampire Pass, there are missions. Missions reset each season. For instance: Kill a certain amount of zombies. Or a certain amount of Wolves. Or kill a certain boss in the campaigns 30 times. Or drop a specific item from a certain campaign. Or drop a specific mission item and hand it over to an NPC. Can literally create countless missions in this game. From as easy as owning 1000 blood packs for a mission, to killing enemies. Kill 20 players. Kill 100. Get 10 Flags. Finish a game of DM or Ctf. Etc. This can give the game an insane amount of content for players.

All this at the cost of only 5 - 10 $.

The sole purpose of the Vampire Pass, is to increase the population of the game. It will definitely bring new players to the game, and push current and old players to play. It can populate certainly empty areas like campaigns, PvP, and so on.