A milestone system would be a good thing to add. Tho instead of a normal "get 25kills and reduce tpa from 30s to 15s" system. i would recommend crying a little and drinking a few coffees to many while making a fully custom system that would include milestones for all the different things that the server gathers info on.
If adding a /milestone command with a UI that pops up, allowing people to view rewards from different tasks.
A kills milestone. a zombie kills. mega kills, resources gathered, farmed items, fishing and quests done. These are just examples to show what could be done and might help maintain a consistent playerbase aswell as enticing people who prefer a more PVE style server but still want to kill people who anoy them.
IF its possible, making so you recieve better fishing rewards or 2x items from fishing 100 fish would be ideal.
less zombie dmg from killing zombies and more dmg to megas from killing megas
A specific milestone where it counts completing the other milestones, so after completing milestone 5 of each milestone, you unlock another /home or something so it makes people want to complete all of them for even better rewards.