Quest system w something like a battle pass / seasonal pass. Users can complete quests and earn rewards, but these rewards are greatly amplified with the purchase of a seasonal pass that unlocks additional rewards. We make it feasible to make an ROI on this investment, but design it so that playtime and active contribution would be necessary. Can require that this user interacts with other users to breakeven on his investment in buying the season pass. Would encourage community building and also spice up the vanilla game.
Run the server as modded server. I currently run a modded server with a custom modpack just for friends and DEFINITELY enjoy it more than vanilla Minecraft. Barrier to entry: potential users would have to download the modpack. Not a complicated affair, we can use public launchers/platforms like CurseForge (https://www.curseforge.com/) or the Technic Platform (https://www.technicpack.net/) to distribute the modpack, but it does add some hassle to the process of joining.