I haven't been around S-G in a while besides popping in every once in a while, but it is a bit sad to see ZE all the way down in ranks. As a former ZE server manager, trying to find a way to keep it populated and balanced between regular and tryhard players was pretty difficult in its own right. I'm not exactly sure what changes have happened since I last played on ZE so I can't really say for certain what the current "meta" is with ZE players these days. However, here's some suggestions that I think would help the server even if a little bit:
Updating existing plugins and finding useful plugins that will enhance the server. Idk how long it's been since some plugins were last updated or added but if they're really outdated its best to go find the latest version for them. I remember there were days where I looked through all the server's plugins and searched for any updates for them because they were just so outdated (like months to even years old). It even helped stabilized the server a bit or fixed some glaring issues or enhanced the server with a new feature. If the plugin is really outdated or isn't being updated anymore, try to find a similar plugin that works better. If you have someone that knows how to make plugins then that's good too. Check the alliedmodder's plugins page for ones that might be worth looking into for the server.
As someone previously mentioned, update the maps and add new ones that are popular among the ZE community. If there are any maps that are broken, plainly bad, or aren't as popular anymore, I would just request a map removal for it and see if others agree to removing the map. As for adding new maps, if you're not sure where to look, Gamebanana has an entire category of ZE maps that are being added/updated every week or so. I also know that some CSGO communities like GFL have their FastDL accessed publicly so you can look and see what maps they're adding or testing on their ZE server.
Events are important. I remember we used to do a lot of events involving beating XYZ map and playing it for a few hours. It always garnered a lot of players and we would have a lot of fun whether we won or not (though beating a map would definitely boost morale on the server). Other events like map testings, holiday themed map events, etc were also pretty fun from what I remembered. Though don't try to overdo them or reuse the same ones every time, you gotta switch it up a little.
Other aesthetic/customization changes like player models and store items helped change the look of the server as well as motivate people to wanting to get them. I know that there were times where I got bored of looking at the same damn model after months of playing tired me out. Every once in a while or during some holiday, we would switch it to something that was appropriate for the time.
Overall, I hope that one day ZE gets back on the top 100 servers. This isn't the first time that the ZE server has dropped down in popularity nor will it be the last. I've seen a lot of ZE managers (including myself) come and go and dedicate their time and effort into making it a fun server while dealing with various technical and player related issues. Listening to everyone's suggestions will definitely help the server and trying to find a balance between what everyone wants for the server will be challenging. For the current ZE managers and players, best of luck to you on getting the server back on its feet because I used to love playing on it almost every day.