So, one of the bigger but perhaps not-so-widely known changes to Minecraft for the 1.12 update was the complete overhaul of the achievements system. The original achievement screen was replaced with a completely new GUI featuring multiple tabs, different tiers of difficulty and a method of adding new, custom achievements to world saves.
It is the last of those points I want to talk about. See, I was looking at the advancements screen, and I noticed that a number of the advancements are simply unobtainable with how the server works ("Subspace Bubble" and "Uneasy Alliance", for example, which both involve nether portals). I also think that a number of us have been feeling the absence of ongoing quests at the moment, and therefore I want to make a suggestion.
Adding custom advancements to the server in replacement of the Vanilla counterparts would create short-term goals for players and therefore promote activity. It could fill that competitive void left by theme months, and, in fact, would provide a more lag-conservative and accessible host for future questlines as they would not require so much reliance on NPCs. I can also confirm that they are server-side; no download would be required for players.
I understand that staff might currently be unfamiliar with the workings of advancements - I myself am not exactly an expert - and therefore implementing this feature would be difficult. However, this board is, as far as I'm aware, open to any and all suggestions, so I thought I'd put this one out there.
If you've made it this far down the post, hats off to you, sir 

Edit: I realise after posting that I didn't really provide any examples of how these advancements would work, so I'll do that now:
A general tab called "Noobstown" or something along those lines would feature advancements such as "Businessman" (Descriptive text: "Order in the Auction!"), which would be received after listing an item on /ah, and "Millionaire" (Descriptive text: "Next stop: 1 billion nbz"), which would be received after hitting 1 million nbz. In fact, you could have a whole advancement tree stemming around /balance milestones, or power levels, or that kinda thing.
Theme month tabs could have more specific advancements. For example, say it's a hypothetical superhero-themed month. Advancements would include "Pacifist" (Descriptive text: "Rid the World of Weapons"), which would involve tossing any sword into a fire, and "Supernoob" (Descriptive text: "It's a bird! It's a plane!") which would be received after flying with the elytra with strength effect applied or something like that. Upon completing all advancements in this tab you might receive a custom head or some other reward along those lines.
These are just a couple of random examples.

+1
Love the idea